hey, im not trying to be a pain, but since the post was to a kid who is assumed not to have taken physics, i should mention something. you said that " longer barrel gives the pressure more time to act on the spud resulting in greater acceleration and..." but it does not result in gretaer acceleration. it results in acceleration for a greater time, and so a greater final velocity. (Vi+(A*T)=Vf, remember?). average acceleration is actually lower, not higher, though the weaker force applied (which causes a lower acceleration) as the spud reaches the additional length of barrel, still adds to the velocity- at least untuill pressure*cross sectional area of the barrel drops below friction, which, if your barrel is REALLY long, will eventually happen. untill then, longer barrels drop the average acceleration but increase muzzle velocity.