The huge weapons on sets such as Jimi Stringer aren't built onto the arm-- they actually are the arm.

What I found most interesting about Stormer (and the other two heroes built like him) was how his massive weapon-thing is built. I had assumed he had two normal arms and this thing just fit over the right one, but nope -- it's one solid, massive piece, which attaches to his shoulder via a basic socket piece. (The same kind he has for ankles.) The "hand" you see in it is molded into the weapon piece.