By B.B. Pelletier
Prechanged pneumatics are all the rage among airgunners these days, because these guns are so accurate and easy to shoot. A spring gun takes technique to shoot accurately. If you try to hold it firmly like a firearm, your accuracy will suffer. But, a precharged gun seems to be as neutral as a firearm, forgiving of different holds to a great extent – but not completely.
When the reservoir fills and empties, your barrel may be moving!
A precharged gun is vulnerable in its reservoir – or, rather, the things attached to the reservoir, such as the barrel. Let me tell you what happens to an air reservoir when it’s filled. Regardless of the reservoir’s composition or how thick it is, every reservoir flexes when being filled.
Although it sounds silly, think of your rifle’s air tank as being a balloon. It may not inflate quite as much when you fill it, but the metal does swell and move. Conversely, as you shoot and exhaust the air from the reservoir, the metal walls shrink and move in the opposite direction. If the barrel is connected to the reservoir at any point, the movement will be transmitted to the barrel and it will move as well.
Low power might cause your groups to shift
A rifle like the Career 707, where the barrel is in contact with the reservoir, will be influenced by the amount of air in the rifle at any point. The saving grace for the 707 is that the two smaller reservoir tubes stacked on top of one another move less than a single tube. Even so, if you shoot it on low power for many shots at long range, you will notice your vertical groups shift slightly as the pressure declines. Since most people buy the Career to shoot at high power, they won’t be affected by this phenomenon. By the time they’re ready to refill the reservoir, the air pressure isn’t low enough to have caused a shift!
Contrast that to a rifle with a full free-floated barrel, such as the Logun S-16. Since the barrel never touches the reservoir, it can’t be influenced by it. Or, so it would seem.
Avoid this common mistake & your groups will improve noticeably!
How do you rest your rifle? Do you rest it on a sandbag? Which part of the gun is touching the sandbag – the stock or the reservoir? A rifle like the Logun Axsor has enough reservoir tube sticking out past the end of the stock to make a nice resting spot for a sandbag, but don’t do it! Rest a PCP on its stock – never on the reservoir tube - so your groups don’t wander. My theory is that the harmonics of the gun change when the metal reservoir is dampened (vibration is stopped) by the sandbag, but that’s just what I think. What I know is that resting a reservoir on a sandbag is a “cure” for accuracy!
Several years ago I caught myself resting PCPs on their reservoirs or their stocks, whichever was convenient. My reservoir-rested groups were horrible, while my stock-rested groups were good. If I hadn’t noticed what was happening, I would have been one discouraged puppy!
Why not conduct your own tests? Shoot several groups with the reservoir resting on sandbags; then switch over and shoot the same number of groups (with the same level of air in the reservoir) while resting your gun on the stock. You should see a difference. Let me know your results!