it seems like a boolean would be the way to go on the muzzle thingy. it can complicate the model alot since it makes some weird edges (sometimes) but it's the easiest way to get uniform holes the 4 places it needs the holes. just make the shape of the holes you need and then use the boolean tool (which i guess you are familiar with)
you make a model of the hole you want and then put it so that it sits where the hole is supposed to be. then you go into compound objects (in the same sublist as the create function) and apply this to your hole model. when you have done this choose the the subtract (B-A A being the holes model and B being the Famas itself) and then pick operand B.
(remember to save the holes model since you may need it again to make the holes again)
i hope this makes sense and if not just google it and there should be lots of tuts showing it.