Muzzle Velocity Question

[CuppaT]

Everything has it's optimum.... longer barrels are generally more efficient, but there is a limit which if crossed the barrel would be less efficient.
So yes, different barrels lengths will need different amounts of air. But barrel length is more than this, it is about being short enough to use in a hide, or long enough to give a better hold. It's different strokes for different folks. Just buy and use what you're comfortable with.

Example: a .22LR is generally most efficient with a 16" barrel, however a .17HMR is generally most efficient with a 20" barrel. Not an example of air rifles but it does go to show that there is more to this than worth worrying about.