Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 6

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.