Hector's Airgun Blog

Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 8

7/22/2021

What happens when you remove the LGU’s muzzle cap?

When first got my LGU, I tried changing as many parameters as possible to see if/how they affected accuracy. First, I tested lots of different pellets. The rifle seemed to like Air Arms Diabolo Field (4.52 mm) pellets, which have tended to be the most accurate pellets in all 12-14 ft-lb rifles that I’ve ever tried. The simplest parameter to change in the rifle itself was to remove the muzzle cap. This greatly increased the report of the rifle, producing a sharp popping sound that is similar to my unmoderated Anschütz 2002 CA PCP air rifle. It also seemed to slightly improve accuracy, so in this chapter I’ll take a closer look at how my LGU performs with and without the muzzle cap/baffle.

Figure 8.1 shows a photo of the LGU’s removed muzzle cap next to the muzzle tube where it was held by a screw. The muzzle cap weighs 36 g (1.3 oz). Since I’m using a computer hard drive magnet to hold the end of the cocking lever in place, I also removed the cocking lever latch. For more info on these mods, please see:

https://www.gatewaytoairguns.org/GTA/index.php?topic=168525.0

Fig. 8.1 Photo of the LGU’s muzzle cap (on left) next to the muzzle tube where it was held by a screw.

Figure 8.2 shows 5-shot groups shot off the bench at 20 yards with the muzzle cap (top six targets on left card) and without muzzle cap (bottom two targets on left card and all targets on right card). Two out of the six groups with the muzzle cap showed significant vertical stringing (see red ovals). The groups without the muzzle cap tended to be rounder. The mainspring used here was a shortened Maccari (moderate FAC, 30 coils x .125 wire) that was shooting around 11 ft-lb. This was only preliminary evidence that the muzzle cap was affecting accuracy.

Fig. 8.2 5-shot groups at 20 yards off the bench for LGU with and without muzzle cap on 8/14/19 using shortened Maccari mainspring shooting around 11 ft-lb.

In January 2020 I installed a 16 J factory mainspring that was shooting around 12 ft-lb. Figure 8.3 shows more 5-shot groups with and without the muzzle cap. The left target card was shot with the muzzle cap in place and the right two cards were shot without the muzzle cap. Most of the groups on the left card looked pretty good, but about a third of them, circled in red, showed the vertical stringing that I observed before. Two things happened when the muzzle cap was removed, as can be seen in the right two cards in Fig. 8.3. First, the POI shifted down and slightly to the left. Second, the groups were rounder with less tendency to string vertically. If anything, there may have been some horizontal stringing going on (see groups circled in green) when the muzzle cap was removed!

Fig. 8.3 5-shot groups at 20 yards off the bench for LGU with and without muzzle cap on 1/20/20 using factory 16 J spring shooting around 12 ft-lb.

Further qualitative accuracy comparisons can be found in Fig. 8.4, where five 5-shot groups were made without the muzzle cap, followed by four 5-shot groups with the muzzle cap, and finally by a single 5-shout group without the muzzle cap. I used the term “muzzle brake” instead of muzzle cap when I annotated the targets. I overlayed the groups on the right side of the target card and found that the 20-shot composite group without the muzzle cap was significantly smaller and rounder than the 20-shot composite group with the muzzle cap, which again tended to be stringing vertically. Even the 30-shot composite group without the muzzle cap looked better than the 20-shot composite group with the muzzle cap.

Fig. 8.4 5-shot groups at 20 yards off the bench for LGU with and without muzzle cap 1/28/20 using factory 16 J spring.

In Fig. 8.5 I try to quantify the accuracy differences better. On the left card in Fig. 8.5, I shot four 5-shot groups, five 10-shot groups, and a 20-shot group, all with the muzzle cap attached. The bottom four 10-shot groups on the left card were made with the muzzle cap removed. On the right card, the top eight 5-shot groups were made with the muzzle cap removed, followed by eight 5-shot groups with the muzzle cap attached, and finally two 5-shot groups with no muzzle cap. I alternated groups with and without the muzzle cap to reduce any artifacts due to accuracy drifting with time. Again, groups without the muzzle cap tended to be smaller and rounder, with a lower POI than groups with the muzzle cap.

Fig. 8.5 5-, 10-, and 20-shot groups at 20 yards off the bench for LGU with and without muzzle cap on 4/9/20 and 4/12/20 using factory 16 J spring.

Figure 8.6 summarizes the accuracy test made in Fig. 8.5. The ctc distances for the 5-, 10-, and 20-shots groups shot with the muzzle cap were at least 20% larger than the groups shot without the muzzle cap. The difference became more significant as more shots were included in the groups, since the vertical stringing by the muzzle cap was intermittent and one could still get some pretty nice 5-shot groups with the muzzle cap attached. However, if one shot more groups or put more shots into a group, the stringing tendencies of the muzzle cap started to become more apparent.

Fig. 8.6 Average ctc group size at 20 yards off the bench for LGU with and without muzzle cap.

Finally, I wanted to check if the accuracy differences would show up in recoil traces. Figure 8.7 shows the rifle’s velocity as a function of time for three shots with the muzzle cap and three shots without the muzzle cap. They all look the same to me! I think this confirms Hector’s diagnosis that the vertical stringing is due to barrel harmonics, which wouldn’t show up in the overall rearward motion of the rifle. If one could zoom in on the muzzle in time and space, one would see that it’s swinging around like the end of a diving board.
During the middle of the swing, the barrel muzzle is moving the fastest and its direction is changing the most. Therefore, if the pellet leaves the barrel when the muzzle is in the middle of its swing, small changes in the pellet exit time and/or the barrel vibration will cause pellet POI to be more spread out laterally. On the other hand, if the pellet leaves the barrel when the muzzle is at the end of its swing, and therefore stopped and is about to turn around to head back in the opposite direction, there will be much less variation in the muzzle orientation and accuracy will be much better.
Removing the weight of the muzzle cap changes the barrel harmonics towards the more favorable situation where the pellet leaves at the end of the muzzle swing. Recent measurements have shown that this is the case. Using a DIY angle-o-meter, I found that with the muzzle brake attached, the pellet leaves the muzzle when the barrel is swinging down, near the middle of its swing. Without the muzzle brake, the barrel oscillations are faster and the pellet leaves near the bottom of the muzzle swing, causing the POI to be lower and the vertical dispersion to be less, since the muzzle is stopped at the bottom of the swing when the pellet exits. For more details on this, please see my post in Shooting the Breeze:

https://shooting-the-breeze.com/threads/barrel-vibration-measurements-in-my-lgu.52757/

Fig. 8.7 Three velocity recoil traces without the muzzle cap (blue) and three recoil traces with the muzzle cap (orange) from my LGU.

The main lesson that I got out of this is that although manufacturers have good reasons for adding every part on their air rifles, sometimes these parts do more harm than good, depending on one’s priorities. I’m happy to live with a louder LGU if it means that is shoots more accurately. I’m very curious to hear if any readers have noticed a difference. Have you tried removing the muzzle cap on your spring piston air rifle? It may be worth a try!

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Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 7

7/8/2021

3 Comments

Note from the Publisher:

Please be so kind as to note the corrections made on July 11, 2021.
Corrections have been inserted in Bold Face.
The chart in fig 7.5 has also been corrected to the proper values.
We apologize for the compound mix-ups and acknowledge the help provided by Britt Salter in identifying the issue.

Does a higher energy spring decrease accuracy in a springer air rifle?

In this chapter we explore what happens when higher power springs are used in my FWB 124 and LGU air rifles. I’ve heard that these rifles are optimized for 12 ft-lbs, but they did quite well with higher energy springs. Let’s start with the FWB 124. When I lubricated my FWB 124 with Krytox, the muzzle velocity dropped dramatically, so I decided to put an older, higher power spring to get the muzzle velocity back up. This gave me the opportunity to test how the FWB 124 performs with lower and higher energy springs. The lower energy spring was a Maccari Slightly Softer spring, which produced muzzle energies around 11.2 ft-lb. The higher energy spring was a Maccari Pro-Mac, which produced muzzle energies around 13.3 ft-lb. I would expect that the weaker spring would result in more docile recoil and therefore better accuracy. Figure 7.1 supports this assumption, with the average of eight 10-shot groups for the weaker spring in Fig. 7.1a) being a bit smaller (although within the error bars) of the average of eight 10-shot groups for the stronger spring in Fig. 7.1b).
The standard deviation and extreme spread of the muzzle velocities were pretty much the same for both springs.

Fig. 7.1 FWB 124 10-shot groups off bench at 20 yards with a) Maccari Slightly Softer mainspring (red rectangle) and b) Maccari Pro-Mac mainspring.

A 2 ft-lb (19%) increase in muzzle energy is significant, so I was curious to see how that increase would affect the recoil traces. I looked at the recoil of the FWB 124 with the two springs over longer times in Fig. 7.2a) and shorter times in Fig. 7.2b). Figure 7.2b) also shows when the pellets exited the muzzle. Surprisingly, the position, velocity, and acceleration traces are nearly identical over the first 0.015s! The main difference is that the later oscillations are shifted slightly to the right (later times) for the weaker Slightly Softer spring (purple traces). These occur well after the pellet has left the barrel, so I doubt that they had much effect on the accuracy or muzzle velocity. Maybe they are just due to the different springs sloshing around differently once the pellet has left the barrel?

Fig. 7.2 FWB 124 recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle for the Maccari Slightly Softer and the Maccari Pro-Mac mainsprings. a) looks at longer times and b) focuses at shorter times and shows the pellets leaving the barrel (vertical red lines).

Now let’s take a look at my LGU. For the past year, I’ve been the only competitor in the World Field Target piston class at the monthly Rochester Brooks matches (https://www.rbgunclub.com/field-target/), so I decided to try the hunter field target piston class. HFT has a muzzle energy limit of 20 ft lbs, so I replaced the 12 FT-LB (16 J) factory spring with a 20 J (15 ft-lb) spring that came with the rifle. The higher muzzle energy produces a flatter trajectory that should really help with accuracy and ranging at longer distances.
Figure 7.3a) shows some targets at 20 yards off the bench with both springs and Fig. 7.3b) shows three targets at 52 yards off the bench with the 20 J spring. At 20 yards, the accuracy was pretty much the same with both springs despite the fact that recoil was significantly stronger with the 20 J spring, which increased the muzzle energy from 11.1 ft-lb to 14.4 ft-lb.
The variation in muzzle velocity was similar for both springs. I was very excited by the accuracy that the 20 J spring produced at 52 yards. The three 10-shot groups at 52 yards are the best I’ve ever shot with a spring piston air rifle past 50 yards. I was especially pleased that the groups drifted only slightly, with the third group back on top of the first group. All the groups had ctc distances under 0.9”, with the first and third groups under 0.75”. Remember, these are 10-shot groups!
The higher muzzle velocity also took off about an inch of pellet drop at 52 yards, which will help reduce misses due to ranging errors. For comparison with 10-shot groups at 52 yards using the 16 J spring, please look at Fig. 5.6 in Ch. 5.

Fig. 7.3 LGU 10-shot groups off bench at a) 20 yards with 16J (top) and 20J (bottom, red rectangle) mainspring and b) at 52 yards with 20J mainspring.

Figure 7.4 shows the recoil traces from my LGU with the 16 J and 20 J springs. Unlike the FWB 124, where increasing the muzzle energy by a couple of ft-lbs didn’t make much difference in the recoil traces, with the LGU a similar increase in muzzle energy made a big difference in the recoil. I aligned the 16 J and 20 J traces using the pellet exit signals (bottom curves in middle plot). The amplitudes of the velocity and acceleration peaks and dips were clearly bigger with 20 J spring. These peaks and dips also were shifted to longer times, suggesting that the shot cycle took a bit longer with the 20 J spring. This is opposite to the behavior in the FWB 124, where the weaker spring shifted the recoil traces to slightly longer times (to the right). Ideally, the traces should be aligned according to when the piston was released, but this is hard to nail down very precisely, so I had to use the pellet exit time, which will be slightly different due to differences in muzzle velocity.

Fig. 7.4 LGU recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle for the factory 16 J and 20 J mainsprings. Vertical red lines show the when pellets leave the barrel. Note that for the 20 J spring only a single light gate was used, so there’s only one pulse in the bottom orange trace in the middle graph.

Finally, I checked to see how the efficiency changes when going to a more powerful spring.
Before discussing the efficiencies, I'd like to thank Britt Salter for catching some important errors in the original version of Fig. 7.5. Thanks Britt for catching the inconsistencies between the posted muzzle velocities and calculated efficiencies in the original version of this chapter!
Figure 7.5 has been corrected and shows that the stronger spring required more work to cock for both the FWB 124 and LGU, but for the FWB 124 the resultant increase in muzzle energy actually increased the efficiency, while the stronger spring in the LGU decreased efficiency. One difference with the FWB 124 is that the weaker spring was lubricated with moly/Superlube and the stronger spring was lubricated with Krytox, so the efficiency comparison is not as reliable as with the LGU, where both springs were lubricated with Krytox. The increase in efficiency with the stronger spring in the FWB 124 isn’t very dramatic, but at least one can conclude that efficiency in this case did not go down when a stronger spring increased the muzzle energy by 19%. I’m willing to live with the decrease in efficiency of the LGU with the stronger spring if it can maintain better accuracy at longer distances.

So for my FWB 124, increasing the muzzle energy from 11.2 ft-lb to 13.3 ft-lb may have hurt accuracy slightly at 20 yards, but not significantly, without affecting the standard deviation nor the extreme spread in the muzzle velocity. The shot cycle at short times was pretty much the same for these two springs, but it looks like the oscillations with the weaker spring were slightly delayed well after the pellet exited the barrel. The efficiency with the stronger spring was slightly better. It would really help to check the accuracy at longer distance to see if the higher muzzle velocity produced by the stronger spring can result in better accuracy at longer distances. The higher muzzle velocity of the stronger string will produce a flatter trajectory, which also will help at longer distances.
For those of you who want to maximize the muzzle velocity of your FWB 124, I suggest that you try the factory piston seal. I was typically getting up to 50 fps greater muzzle velocity with the factory piston seal over any aftermarket seal that I tried using a variety of mainsprings.
For the LGU, the stronger spring decreased efficiency a bit and didn’t change accuracy at 20 yards. However, at 52 yards the accuracy looks very promising (personal best!) and the flatter trajectory will certainly help ranging and hitting targets at longer distances. The recoil felt stronger, which is backed up by the recoil traces, but that didn’t seem to hurt accuracy. If it weren’t for the 12 ft-lb limit in the World Field Target piston class, I would definitely use the stronger 20 J spring in those matches. Since the muzzle energy limit in the American Airgun Field Target Association’s Hunter piston division is 20 ft-lbs, I’m looking forward to trying the stronger spring in my LGU for hunter FT matches. I wonder if there’s anything that can get me a few more foot pounds out of the LGU?! It also would be interesting to see if the efficiency continues to drop in the LGU with even stronger springs. We expect that the shorter, central transfer port in the LGU should work better at higher power compared to the longer, offset transfer port in the FWB 124, but at least for this particular test, the FWB 124 did a bit better in terms of efficiency at higher power.

3 Comments

Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 6

6/24/2021

5 Comments

Does more mass in a springer air rifle result in better accuracy?

Since an air rifle moves a fair amount before the pellet leaves the muzzle, it makes sense that anything that can inhibit that motion could help improve accuracy. An easy way to decrease movement is to add mass to an object. Newton’s famous 2nd law of motion states that F=ma, where F is the force applied to an object, m is the mass of that object, and a is the resulting acceleration of the object due to the force. For a given force, increasing the mass will decrease the acceleration, i.e., a=F/m. It is not too surprising that the accuracy ranking of the LGU (16.3 lbs), FWB 124 (14.3 lbs), and LGV (13.1 lbs) follows the masses of these three rifles. Of course, the difference between first and third place ctc averages is less than a ¼ of a pellet diameter, so the accuracies are quite close. However, after shooting thousands of pellets in these rifles, I think that this ranking is robust. However, these are three different rifles that have differences that go far beyond just their masses. So in this chapter, I will explore how the same rifle, my LGU, behaves when its mass is changed.
Changing the mass in my LGU was easy; I just switched from the homemade heavy FT stock to the original factory stock. The total weight with the FT stock is 16.3 lbs while the total weight with the factory stock is 12.7 lbs. The LGU is bedded in the FT stock with bedding compound and aluminum pillars, so that may help with day-to-day consistency compared to the wood-bedded factory stock.
Also, the beefier FT stock is a bit easier to hold on the sandbags. Despite these advantages, I think that the main difference is just the difference in overall weight.
Figure 6.1 shows the LGU in the a) FT stock and b) the factory stock.
The four 10-shot groups at the top of the target card in Fig. 6.1c) were made using the heavy FT stock while the four 10-shot groups at the bottom of the target card in Fig. 6.1c) were made using the factory stock. I went back to the FT stock for the 10-shot group at the very bottom of the target card. The groups weren’t great on that day and I’m not sure what was causing the vertical stringing. The main point is that the groups shot with the heavy FT stock were very close to those shot with the lighter stock. It’s interesting that the lighter stock caused a POI shift down and to the right. It’s very reassuring the POI returned to the same place when I put the LGU back into the FT stock for the target at the very bottom of Fig. 6.1c). The average ctc distance for four 10-shot groups using the FT stock was 0.28”±0.09” while the average ctc distance for four 10-shot groups using the factory stock was 0.29”±0.01”.

Fig. 6.1 LGU in DIY FT stock a) and original factory stock b). c) 10-shot groups at 20 yards from bench using FT stock (first four groups and bottom left group) and original factory stock (bottom right four groups). Total weight was 16.3 lbs for FT setup and 12.7 lbs for factory setup

Figure 6.2 shows the recoil traces for the two configurations. Not surprisingly, the recoil with the FT stock was reduced, with smaller peaks and dips in the position, velocity, and acceleration of the rifle. Since the moving sled itself weighs 2.4 lbs, we really are comparing a total recoiling weight of 18.7 lbs with 15.1 lbs (not the bare rifle weights). This translates in a weight decrease of about 20% in going from the FT stock to the factory stock. The decreases in the first velocity dip and first velocity peak were around 28% and 25%, respectively, which are a little more than what I would have expected if we simply scale the recoil by the rifle/sled mass. The greater mass also seems to smooth over some of the oscillations that occur in the 0.03s to 0.05s time range.
At longer times, the recoil traces come back together to be almost exactly on top of each other! One might expect the recoil dynamics to be slowed by the extra mass, but except for the second peak in the velocity, there don’t appear to be any clear shifts in time. It’s also interesting that the piston bounce time (when the velocity goes to zero and the rifle stops its rearward motion for the first time) is the same for both cases.”

Fig. 6.2 LGU recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle over 200 ms for a lighter factory stock and a heavier FT stock. Note that velocity was measured using the soundcard on a pc, so the signal is ac-coupled and the slower rearward drift of the sled is not captured.

So if I were shooting groups from a bench I would definitely go with the heavier FT stock. If I was in a Field Target competition where the fit of the stock is important and the extra weight helps steady the rifle, I would also take advantage of the FT stock. The advantages of the FT stock go far beyond just adding weight. The FT stock has an adjustable height cheekpiece (which is critical for the very high scope mount that I’m using), and a buttplate that can be moved forward and back as well as up and down. This allows one to fit the rifle to the shooter and to the particular shooting position. A good fit allows one to relax and let the rifle rest with very little muscular effort. This makes it much easier to shoot the rifle accurately even if the intrinsic accuracy is not greatly changed. However, if I was carrying the rifle for longer distances, the lighter factory stock would make more sense and would produce similar levels of accuracy as the FT stock.

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The DIANA 34 EMS (Easy Modular System).- Chap. 1

6/13/2021

10 Comments

A new breed of Break-Barrel Piston rifle

Back in 2016 (that is already 5 years ago!) When I met Tobias Schmidt for the second time at Pyramyd Air Cup, we started toying around with ideas for the future of DIANA.
During the long hours of talks and idea sharing, we developed a respect and friendship that I am glad to be able to say that has survived the changing circumstances of life.
In subsequent years, we attended several IWA's and PAC's and our long drives to and from those events were usually spent brainstorming about the products and the markets.
About 2018, when we drove from Maryland to Ohio together, we started discussing in earnest what the EMS COULD mean to the shooters. Before that, it had been mainly a manufacturing simplification system, where the products could be built using common parts and only some parts would differentiate the versions.
Through our conversations, Tobias saw the value that the system could bring to the market; specifically, the US market that has a long tradition of "home tinkering" and enjoys a freedom not granted to citizens of other countries to CREATE our own rifles.
And so the "Easy Manufacturing System" became the "Easy Modular System" with a focus more towards the user than to the maker.

Given the HUGE opportunity that designing a new platform brought, we looked at every detail and every nook and cranny of the rifle, from the sights to the butt plate.

And when we were almost ready to release the first pre-production steps (first one series of 10 rifles, then a series of a 100), to then go into production at full pace, CoViD struck.

This disease/pandemic has brought different responses from different countries. MOST have opted to limit people's mobility and contact, some have opted to do nothing beyond putting the responsibility in the citizen's shoulders. Others started with a lenient attitude only to go into panic when hospitals filled overnight.

But in all aspects, it has been mostly "social distancing" what has prevailed.

In a production environment, "social distancing" carries a lot of collateral effects.
Things don't get transferred as fast and as completely as when there is face to face contact.
Things take, at least, twice as long (if not four times as long).
There are more errors and more problems that could have been solved, but were not, because people are not REALLY working together.

All our plans to release the whole suite of parts and accessories simultaneously with the guns have gone out the window. Though SOME parts are ALREADY available. And the reason is simple:

EMS builds onto the excellent results of the NTec series by using the NTec piston and trigger block; and then enhancing the architecture of the gun completely.

There are three "reviews" of the EMS currently published:

https://hardairmagazine.com/reviews/ham-exclusive-first-diana-34-ems-test-review/

https://www.airgunsofarizona.com/blog/2021/02/theres-a-surprise-at-aoa-the-new-diana-34-ems.html

https://airghandi.de/en/diana-34-ems-air-rifle-my-test-and-review/
This last review has a video (in German), that is quite understandable even if you do not speak German.

As good as the reviews are from a consumer perspective, there is a LOT more going on and I will try to touch on some points that seem to me to be of importance, yet ignored by the reviews in favour of velocity numbers and pellet tests. IF there is enough interest, perhaps we can post a complete analysis of the differences and the real advantages of the new architecture.

A typical complaint in the fora, usually by people who are "keyboard shooters" more than the real thing, has been that the parts to upgrade the guns are not yet available, but that is not completely true, and in this blog entry we will look into an upgrade that CAN be performed with a part in the current list of spares from DIANA.

From Steel to Gas Spring

As we have mentioned above, the EMS builds on the success of the NTec rifles (mainly the 340 NTec) by using the NTec piston (that is stemless), and the T-06 NTec trigger (different from the T-06 Stemmed piston trigger).

But, let's get started:

This is a 0.22" cal 34 EMS as they come out of the box.
The "Classic" version of the stock is the one currently available. IF you compare it to an old 34 "Classic" you will see that this stock is slightly more slender, and that the comb is a bit higher
This puts the recoil more in line with the center/shoulder point of contact to the butt plate, and so, the shootability of the gun gains because there is less "jump" to the gun when the piston is released.
This is something that has already been identified by HAM's technical reviewer, my friend Eric Brewer.

The front sight is a modern unit with fiber optic "pipe". It is held in place with the traditional grub screw from the top and a nut at the front. And NO the synthetic cover is NOT the holding nut, it is a threaded cover and thread protector of the ½"-20 UNF thread at the barrel's muzzle.

This is the real nut. Steel. It takes a 15 mm's slim wrench to losen it properly.

The rear sight is an interesting element.
With dual fiber optic pipes, it is adjustable for both windage and elevation and the screws have re-assuring "clicks" that tell you that the screws will not move on their own, as well as allowing to count the variation you are incorporating in the LOS.
You can also appreciate here the fork adjustment screw and nut, they both take a 5 mm's Allen wrench. Screw is on the left hand side, nut is on the right, and underneath are real nord-loc washers. IF your forks are "Too tight" shoot the gun at least 100 shots before deciding to losen the arrangement. In any case, you do not need to take the stock out to adjust the breech-block to fork tension.
Another thing to note is that all 34 EMS's are issued with the Serial Number in TWO locations: at the fork as you can see from the picture, but also in the barrel, so that people can know if a gun is keeping its original barrel or not.

As you can see, the sight picture is quite usable. Shooting with both eyes open is possible and reasonably accurate.

With the sights removed, now we can remove the stock and put the gun in the spring compressor.

To remove the stock there are two forearm screws and a trigger guard/block screw. In the picture above, you can see that the forearm screws are ANGLED into the compression tube finishing block. Something that is shared with some of the best airguns in history.
For the savvy shooter, this allows some degree of "tuning" because the tension between stock and action can be regulated in a solid manner. This could also be done with the "old" 34, but now the degree of adjustment is one degree of freedom better.

The EMS has a compound cocking linkage, which was first used in the 350 Magnum, the compound linkage reduces the Peak Cocking Force (PCF), AND allows an easier disassembly/piston extraction for maintenance or upgrade purposes.

This is what you find in the underbelly

This is how the linkage pops-up

And you simply remove the linkage from the piston. That allows the piston to be removed without having to disassemble the whole linkage from the barrel, as used to be the case with the "old" 34.

This is the T06NTec trigger block. You can clearly see the steel main spring and the rear guide inside the main tube.
The T06Ntec trigger is an interference trigger. Meaning that there is a ramp that rises and locks into the slot at the skirt of the piston. When the trigger is released, the ramp "falls" out of the way.
It is an incredibly fast trigger, especially useful for offhand shots.

Once the rear dust cover is removed, and the compressor has taken all the stress from the closing pins, the dummy pins can be inserted and the trigger unit can be slid back to reveal the spring, and the rear guide.
Removing the whole spring lets you access the front guide.

This is what you have to get out of the gun, from left to right: Front guide, spring, rear guide.
The front guide is a solid piece of steel, and adds considerable weight to the piston. Once that guide is removed the gun's power plant is "skewed" towards lighter pellets.
This front guide can be replaced with an Anti-Bounce unit, but that is a custom project we may talk about in a future entry.
In the trigger you will find a simple allen headed screw that needs to be removed.

Here you can see the screw that acts as solid support for the rear guide. You can also see in the bottom of the trigger unit the "ramp" that locks into the piston slot, when that ramp falls, the piston flies forward.

Once that screw is out, you can then simply screw the NTec gas spring into the trigger unit.
I would STRONGLY recommend using Vibra Tite by applying to the thread and then letting it dry. THEN assembling the NTec gas spring to the T06NTec trigger block.

The "button" at the end of the stem in the gas spring needs to fit into this recess in the piston's head.

And you need to ensure that everything is properly aligned.

When everything is a go, you simply insert the trigger/gas spring assembly, properly seated into the piston, into the gun. I STRONGLY recommend using shim material to prevent the cuts in the mechanisms tube from scoring/cutting/hurting the piston seal.

If you want to de-burr the edges prior to assembly, do use the "quintessential" tool of rifle-making. It's called a "riffler file" and nowadays the diamond versions are not that expensive. Get a proper one and learn to use it (rifflers are used by pulling, as opposed to files that are used by both pushing and pulling). It will save you a lot of headaches.

Put everything in, pop the cocking linkage in its place, put the gun in the spring compressor, compress the gas spring, put in the action closing pins, put the dust cover.
Assemble the stock to the gun, taking care to glue some flat washers to the bottom of the recesses so as to protect the wood from the star washer chewing the stock to death.

And test.

With the NTec gas spring, it takes a meager 30½ lbs of PCF to fully cock the gun.
Testing different pellets at short range will give you an idea of which pellets to try at long range.

The three pellets that are outstanding in this particular example are the JSB's 16's, the 14.3's and the GTO's at 11.75 grs.
The Crosman, FTT's, and other pellets simply did not "jive" with this barrel. Perhaps in the future, once the full 500 shots "running in" has been accomplished things may change a little.

While the JSB 16's behaved better, the GTO's perhaps need more work.
The JSB's yielded 15 ft-lbs, while the GTO's yielded upwards of 17 ft-lbs

So, for "real-life" (meaning lead) pellets the conversion complies with the catalog specs, using the GTO's (which are not bad as a hunting pellet out to about 40 yards), it easily exceeds the catalog specs.
This rifle is now equipped with a fixed shroud that still allows the tuning of the muzzle harmonics, so once the user defines exactly what he wants to do with the gun, some fine tuning is possible to achieve the best accuracy with the pellet he chooses.

While the rifle may seem a bit too long, remember that there will be short barrels available in the future.
In our tests, the length of the barrel is not as crucial to the energy delivered to the pellet as in the PCP's.

The conversion itself took all of 20 minutes to do and I think it is achievable by anyone that can change a spring or a piston seal. I do believe that it complies with the "design intent" when we set-out to make a gun that was easy to be "converted" to what each shooter wanted.

As this pandemic eases and we learn to live in the "new normal", I hope that more and more of the planned accessories and tuning elements will become available.
For the time being, I have secured 8 gas springs to do this conversion, if you are interested, drop me a line through the contact page.

Keep well and shoot straight!

HM

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Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 5

6/10/2021

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Does lubrication with Krytox improve performance?

There are lots of excellent articles on the benefits of lubricating spring piston rifles with Krytox, but I’ve never been able to find a detailed, quantitative before/after evaluation of Krytox in these rifles. There also are some excellent posts on Krytox in GTA:

https://www.gatewaytoairguns.org/GTA/index.php?topic=141898.0

I first heard about Krytox from Leo Gonzales. Leo is a fantastic WFTF competitor (he placed 4th in the last WFTF World Championship!) and he knows a lot about springers, so when he makes suggestions, I listen very carefully!
In this chapter I switched my LGU and FWB 124 to Krytox and took a look at their performance before and after the switch.
Before Krytox, both rifles were sparingly lubricated with a combination of moly and Superlube, with the moly mainly for the metal parts and Superlube for the synthetic piston seal. This was the lubrication used in Chs. 1 and 2 of this blog.
Before lubricating with Krytox, I first thoroughly degreased the mainspring, piston, piston seal, and compression chamber. Then I applied GPL 205 very sparingly, wiping off after applying. Krytox is much more expensive than molybdenum disulfide or Superlube, but Leo pointed me to an online store with pretty reasonable prices:

https://www.amazon.com/Krytox-Grease-Pure-PFPE-PTFE/dp/B00MWLDALQ/ref=sr_1_16?keywords=krytox&qid=1580829181&sr=8-16

Figure 5.1 shows how Krytox affected the accuracy of my LGU. Accuracy was about the same after Krytox was applied, where four 10-shot groups were made off the bench at 20 yards before Krytox, and another four 10-shot groups were made after Krytox. No warm up shots were taken after the Krytox lubrication and the last 10-shot group with Krytox is pretty nice, with a ctc distance of 0.090”, so maybe accuracy is improving as the rifle starts settling with the new lube? As I discussed in Ch. 3, this group needs to be taken with a grain of salt!

Fig. 5.1 LGU accuracy with 10-shot groups off bench at 20 yards with moly/Superlube (top four groups) and Krytox (bottom four groups).

Accuracy with the two types of lubes was similar, but there was a BIG difference in the fluctuations in the MV. Figure 5.2a) shows the MV for each of the first forty shots in Fig. 5.1, before Krytox. Figure 5.2b) shows the MV for each of the last forty shots in Fig. 5.1, after Krytox, plotted with the same vertical scaling. The standard deviation and extreme spread of the MV both dropped by about a factor of two when the moly/Superlube was replaced by Krytox! Again, this happened right after the Krytox was applied; there were no shots to get the rifle settled in with the new lube. My LGU shoots lots of different types of pellets very well with similar POI, but one of my concerns has always been that the MV can be pretty erratic with some brands/dies of pellets. At 20 yards, fluctuations in MV aren’t that critical, but at longer ranges these fluctuations could cause big POI shifts. The Krytox did an amazing job of stabilizing the MV using pellets from the very same tin.

Fig. 5.2 LGU MV with a) moly/Superlube and b) Krytox for the groups on shown in Fig. 1.

Krytox made no discernable difference in the recoil of the LGU, as can be seen in Fig. 5.3, which shows recoil traces before (blue) and after (orange) Krytox.

Fig. 5.3 LGU recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle over 250 ms for moly/Superlube and Krytox lubrication of piston seal and mainspring. Note that velocity was measured using the soundcard on a pc, so the signal is ac-coupled and the slower rearward drift of the sled is not captured, as explained in Ch. 1.

Later on the same day, I shot eight 10-shot groups off the bench at 20 yards with my Krytox-lubed LGU, as shown in Fig. 5.4. These are the best 10-shots groups that I’ve ever shot at 20 yards with any air rifle, with an average ctc of 0.23”! MV fluctuations were a bit higher than I would have hoped, but I expect that if I had been using the original moly/Superlube on that day with those pellets, the MV fluctuations would have been even worse

Fig. 5.4 LGU accuracy with 10-shot groups off bench at 20 yards with Krytox.

Since Krytox did such a good job of taming MV fluctuations, I did some accuracy testing at 52 yards where MV fluctuations make a much bigger difference than at 20 yards. At 50 yards, a decrease in MV from 800 fps to 760 fps translates into a drop of around 0.75”! The extreme spread in MV using my LGU with Superlube/moly is around 37 fps, so one will get vertical stringing on the order of 0.75” at 50 yards just from MV fluctuations. Figure 5.5 shows typical groups at 52 yards off the bench before Krytox. Looking at the bottom row we can see that some 5-shot groups were pretty small, around 0.40”! However, there was a lot of vertical drift in the first two groups, shot from left to right, so I only counted the last three 5-shot groups. Again, choosing the smallest group would not be very representative of how the rifle was shooting. Putting the last three groups together resulted in 15 shots inside a 1” circle.

Fig. 5.5 LGU accuracy with 5-shot groups off bench at 52 yards with moly/Superlube (before Krytox). Here I’m focusing on the last three groups on the right in the bottom row.

Figure 5.6 shows how the Krytox-lubed LGU did at 52 yards. Unlike Fig. 5.5 where I shot three 5-shot groups, in Fig. 5.6 all the groups were 10 shots. I was hoping that the Krytox would results in a more dramatic improvement at 52 yards, but the groups are still pretty good for a springer at 52 yards! Most 10-shot (not 5-shot!) groups were under 1” and 70 shots landed in a circle just over 1” in diameter.

Fig. 5.6 LGU accuracy with 10-shot groups off bench at 52 yards with Krytox. The first group in upper left hand corner was for warming up and wasn’t counted.

My LGU’s POI has always been pretty consistent and I think part of this is because I use very little lubricant. I’m not sure if the consistency is much better with Krytox, but I’m finding that over several months under different conditions, the POI has not changed much.

Encouraged by what Krytox did to my LGU, I decided to try it in my FWB 124. Surprisingly, and unfortunately, Krytox had a very different impact on my FWB 124! With the Krytox-lubed LGU, MV remained about the same but MV fluctuations dropped by a factor of two. With the Krytox-lubed FWB 124, MV dropped by about 35 fps and MV fluctuations stayed about the same, as can be seen in Fig. 5.7. Figure 5.7 a) and b) show that accuracy was about the same or perhaps a bit worse with the Krytox. Maybe it will take some time for the rifle to settle in with the Krytox? One possible explanation is that my FWB 124 was lightly dieseling with the moly/Superlube, which have a lower ignition temperature than the Krytox, so that pushed the MV up a bit? That’s just one guess.

Fig. 5.7 a) FWB 124 accuracy with 10-shot groups off bench at 20 yards with moly/Superlube (top four groups) and Krytox (bottom six groups), b) more 10-shot groups with Krytox. Average MV dropped about 35 fps but the standard deviation and extreme spread stayed about the same when Superlube/moly was replaced with Krytox.

So is Krytox worth it? If I had only tested my LGU, I would answer with an emphatic YES! However, the Krytox experiment on my FWB 124 showed that things are a bit more complicated, as they tend to be in real life!
I’m very intrigued how/why Krytox could affect two 12 ft-lbs springer so differently. Maybe it has something to do with the differences in piston seal material? I would very much like to understand the cause(s) of these differences and believe that will teach me something new about springers. If you have any ideas, please let us know.

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Hector Medina

2012 US National WFTF Spring Piston Champion
2012 WFTF Spring Piston Grand Prix Winner
2013 World's WFTF Spring Piston 7th place
2014 Texas State WFTF Piston Champion
2014 World's WFTF Spring Piston 5th place.
2015 Maine State Champion WFTF Piston
2015 Massachusetts State Champion WFTF Piston
2015 New York State Champion WFTF Piston
2015 US National WFTF Piston 2nd Place
2016 Canadian WFTF Piston Champion
2016 Pyramyd Air Cup WFTF Piston 1st Place
2017 US Nationals Open Piston 3rd Place
2018 WFTC's Member of Team USA Champion Springers
2018 WFTC's 4th place Veteran Springer
2020 Puerto Rico GP Piston First Place
2020 NC State Championships 1st Place Piston