jackssmirkingrevenge wrote:What about the lack of air resistance in the barrel as the projectile accelerates?
Large scale laboratory guns with evacuated barrels are something I *do* have experience with. Well... I have experience with at least one such gun.
Basic info on said gun....
Bore: 40 mm
Barrel: 25' or so (don't recall exactly)
Muzzle Velocity: Mach 6+
Basically, we have pre-set loads that give us certain muzzle velocities. When we lose vacuum in the barrel, our velocities drop about (from memory) 5%.
But what's interesting is that if you look at the numbers, you find that the gun's performance is VERY constant.....
...If you look not at muzzle velocity, but rather muzzle energy.
You just need to account for the air mass as well. In other words...
ProjMass * GoodVel^2 = (ProjMass + AirMass) * BadVel^2
That relationship fits our test data almost perfectly. So, at least for high speed guns, it's very easy to predict the performance boost (or not) of pulling a vacuum on the barrel... Just calculate the mass of the air in the barrel and compare it to the projectile mass. If the air mass is negligable by comparison, then so too will be the affect of vacuum in the barrel. If the air is significant... Well, so too will be the affect of vacuum.