Algodoo

your gun is fine the only thing you need to change is the firing mechanism, try to make one like a 99 restitution retangle in the lower part of another normal, noncollide retangle, like an "l_" where the underline is the 99 restitution rectangle. then add a hinge in the top part of the "|" don't forget: autobrake and inf. strenght!,
it's kinda hard to explain... hope it helps!
sorry my english, i'm from brazil =)