How do I enhence the power of a air rifle without changing the projectile?

Answer 1

Higher pressure will result in a higher-velocity pellet.

Answer 2

Its true that you could increase the pressure of the CO2 canister or whatever is storing the propellant (maybe by heating it) but this could also blow up in your face. I would try looking at how the air is pushed into the gun and see if you could remove any resistance to flow. More flow will get you more velocity.