Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 8

​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.
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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:
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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!