AH-HAA! That's what you want pawls for. I have to say, very nice work so far, and a very interesting idea. I've almost perfected the .68 cal upgrade to my receiver and should be able to do work for ya in September. Ive just got a problem with breaking certain types of paint I want to fix.
What exactly are the purpose of those holes? Ive got SMG bodies with and without holes in the bottom of the breech. Assuming those are the holes you were talking about.