I think all spring guns have the reverse piston action, (some recoil-less guns have an action that slides inside the stock, but the spring still does it thing) it takes a split second for the piston to whip forward and the pellet to leave the barrel. If you shoot with poor or inconsistent form the time it takes for the pellet to leave the barrel can cause inconsistencies. Sounds much worse than it really is for me. I just put the X on the critter and watch the pellet hit. If you do that, your follow through is good and there is little torque in your system.
One other thing that happens quite often is that guys will try to shoot them from a solid rest. As noted above with the artillery hold, a spring gun likes to move when fired. I have had excellent luck using cross sticks with a leather pad or when shooting groups resting the gun on a rolled up sleeping bag. Those rests tend to allow the gun to move but also provide a nice platform to shoot tiny little groups from. Rule of thumb: The more powerful the gun, the more difficult it is to shoot and like everything else, the nicer (more expensive) guns have better triggers, smoother actions etc.
