Newton's Third Law of Motion (every action has an equal and opposite reaction) applies to precharged air rifles as it does to everything else. The reason you don't feel it at low power levels is that it is so small. it is related to the energy in the pellet so as pellets get heavier and MV gets faster the effects begin to be noticed. In .22 I notice it around 40 ft lb with 21.4grain Bismags. In .25 I certainly notice 31 grain Baracudas being spat out of the rifle at 55 ft lb. In .177 the only time you would notice it would be when using very heavy pellets at 30+ ft lb.