I had an airsoft AR-15, which is pretty much the same gun, so I can say from experience that that's one of the most accurate CG guns I've seen. Once textured it should be even amazinger

Are you gonna be modelling the little release pin at the back of the barrel in? (Assuming the M-4s have these, the AR-15 does) If you ever have a shot of it opened in the final renders, a little detail like that would add a ton of realism (to me anyways, though most people aren't gun nuts )

Love this project BTW