One suggestion I have though, if you want the rounds to eject backwards a bit more you can put the ejector where the edge hits the bottom of the bullet.
You can't transfer this gun to a different scene, because it uses scene.my.variables. So you just have to modify it on this scene and bring other things into it.
Wow! the skin is absolutely amazing, as well as the inside.
One thing I would suggest though is increasing the simulation frequency to around 300 and increase the density on some things too. That makes it a lot more stable and robust.