Well, that would be a function of the speed of the projectile.jeepkahn wrote:And my question is how much does that affect acceleration???
In a normal barrel, as the projectile speeds up, the air in front of the projectile compresses more and more until the point where the projectile is nearing the speed of sound. At the speed of sound, a shock wave will form in front of the projectile. This shock wave will exert a very high pressure on the projectile, opposing your chamber pressure. This is why it is very difficult to fire a projectile at speeds greater than the speed of sound.
In the case where the barrel has a vacuum, there is no air resistance ahead of the projectile, no matter what speed it is going. It is thus free to go as fast as your chamber pressure will allow, taking into account projectile mass, wall friction, and max fluid flow through you barrel diameter.
If you are looking for equations, I'm sure I (or someone else) can locate and post them. Isn't fluid flow fun!