Interpreting Muzzle Energy?

[Kilash]

No problem, I enjoy doing the math for this kind of stuff.

A way to measure friction on a projectile inside the barrel is to invert the gun, and place weights down on the projectile through a ramrod until it moves. The weight would be the static friction force for that projectile. Friction I think for a large diameter build would probably not affect your muzzle velocity that much, but for a barrel thats smaller than 1" you probably want to account for it, especially for a very tight projectile, since the force involve is much lower, which for your setup at 250 psi, a 1/2" diameter only generates 49 pounds of initial force.