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Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 8

7/22/2021

9 Comments

 

​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
Picture
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.
Picture
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!
Picture
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.
Picture
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.
Picture
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.
Picture
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/
Picture
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!
9 Comments
RidgeRunner
7/30/2021 04:44:28

I have to keep a close eye on you. At times it might be forever before you put something out and then all of a sudden you become prolific. I have a lot of reading to do. ;)

Reply
RidgeRunner
7/30/2021 15:53:36

Never had an LGU. Something you might try is find thick o rings that fit tight in the muzzle chamber. You may even alternate a few thin and then a thick to create "baffles". Baffles tend to act as air strippers, allowing for a more stable pellet exit.

How they will likely affect harmonics, I have no clue. the soft material may dampen them, however the extra mass may exaggerate such.

It certainly seems to not like the cap.

Reply
Hector Medina
7/30/2021 16:15:49

The half dozen or so LGU's I have tuned have responded very well to a "harmonics reducer" which is, essentially, a floating weight with limited displacement possibilities.
BUT John's LGU did not react as expected, so he decided to stick to the "open cavity".

Thanks for reading!





HM

Reply
bberg7794
8/11/2021 00:35:12

Thank you for suggesting removing the muzzle cap and lever latch! I removed both from my LGU and shot it this evening. The results look promising, though more groups will be required. Removing the cap has the added benefit of making it easy to clean the barrel with a PatchWorm, which is worthwhile in and of itself if the removal doesn't detract from accuracy. I have a thread going in AGN where I mention your article and removing the cap. Though I probably will not take the time to do so, I can imagine turning a long muzzle cap and then after shooting, gradually shortening it to attempt to find the desired weight to get that pellet out the end of the barrel at the exact peak of an oscillation. This probably would be easier of one could film the oscillations. Thank you very much for all of the great articles. A lot of my inspiration to find one of the last LGU's is due to your writing. Thank you again! Brian

Mike Kiser
9/30/2021 18:49:20

Do you still have the HMO pistons and tune kits for the TH56?? I'm in dire need.

Thanks
Mike K.

Hector Medina
8/11/2021 09:56:39

Brian;

All credit goes to John C. who wrote the articles, shot the groups and prepared all the experiments.

As you can see in the comment above, John and I do not agree on how to tune an LGU, but that does not mean we are at odds. It just means that in airguns there are very very few "laws" (so far I have found only ONE).

Test and re-test; come to the statistically significant 10 X 10 shot groups, and you will have an answer that is 98% right. I know the chapter in statistics got overlooked by many, but IF there is one thing in that chapter is that the BEST way to spend 100 pellets is in shooting 10 X 10 shot groups.
;-)
Keep us posted!






HM

Reply
bberg7794
8/11/2021 13:45:04

10 x 10's. I like it! So far, I have shot a fair number of 10-shot groups at 50 yards with three different pellets, but I am going to have to buckle down and begin 10 x 10's with one type of pellet at a time and maybe a barrel cleaning and seasoning between (superstition?). I am still learning how to shoot this rifle and yesterday evening, my last three 10-shot groups (I happened to switch to Air Arms DF for those three) were significantly better than most prior groups. Was it me learning this rifle or is the AirArms pellet actually noticeably better? More groups will tell, hopefully. I think all groups will shrink a bit and flyers will be reduced when I replace the trigger, which I am planning to do.

Bye for now!

Brian

Reply
John C
8/11/2021 12:47:47

Dear Brian,

Thanks for your thoughtful post! You made my day! It would be very interesting to add more weight to the muzzle as you suggested, and knowing how the barrel oscillates could provide some guidance with that. Please take a look at Fig. 5 in my post on Shooting the Breeze (https://shooting-the-breeze.com/threads/barrel-vibration-measurements-in-my-lgu.52757/). You can see that without the muzzle weights, the pellet exits at the first big dip in the barrel oscillation, after the first peak. If you add enough extra muzzle weight you could slow down the oscillation sufficiently to move the pellet exit to the top of the first oscillation peak, another good place for the pellet to exit. In that case, you should find a higher POI with similar improvement in accuracy (less vertical dispersion) as with the pellet exiting at the bottom of the first big dip. I don't think it would take too much more weight added to the baffle and latch weights. It's a great idea and I wish I had thought of it! I just may try this myself to see if I can get the pellet to leave at the first peak, before the first big dip.

Please let me know how you're holding the cocking lever in place. Are you using a magnet to hold it to the barrel, or is it not touching the barrel at all? This could also change how the barrel vibrates.

Thanks and best wishes,
John

Reply
bberg7794
8/11/2021 13:19:11

Hi John,

I used electrical tape to attach two small rectangular neodymium magnets to the bottom of the muzzle tube about 3 1/2 inches aft of the muzzle. I had some kicking around my shop at work and didn't know if they would be strong enough to prevent the cocking lever from detaching at the shot, but I shot my rifle approximately 135 times yesterday evening and the lever stayed secure the entire time. I have a picture posted of the muzzle mod. on Airgun Nation under, "Spending some time with my new LGU."

Regarding fig. 5 in the thread you mentioned, I read that thread a couple weeks ago, but will have to take another look at it when I have more time. Since you are able to measure oscillations over time, you are "seeing" what is going on and could use these measurements for tuning, as you mention above.

Thank you for all of your work and making it public.

Brian

Reply



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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
    2022 Maryland State Champion WFTF 
    2022 WFTC's Italy Member of TEAM USA 2nd place Springers
    2022 WFTC's Italy
    2nd Place Veteran Springers

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