• the magazine (and thus the receiver, but mostly the magazine) which is connected to the barrel and supplies bullets to shoot
May as well just be the reciever, problem I always see in gun designs is how the ejection port's in the wrong place, there's not enough space for the firing action, the safety and fire mode dials are in funny places, etc etc. Magazine, while prominent in its visual, are mostly just the same: Boxes to put rounds in. In the end all they might dictate is which direction you load them in from.
• the barrel is almost always the thinnest part of the gun: in other words, the rest of the gun should be fatter then the barrel. the only case this is not true is with grenade launchers, and it is still the exception rather then the rule
If the barrel is larger than the reciever and trigger group that could mean the round your firing fits into the trigger group but not the barrel. When your bullet can't fire out of the barrel you have a problem.
I guess Bushido's talking more of an aesthetic standpoint. I guess you should talk to him if you want to draw some fanart for Borderlands or something.
WE CAN BE THE FIREARMS DESIGN OVERSEER SQUAD