Looking slick! I'm a bit over the whole steampunk thing myself, but this is a nice piece, regardless.
More than anything, I'd recommend doing something with the cooling jacket (on the barrel).
A lot of old-school machine guns were water cooled- maybe ding up the jacket a bit, add rust/corrosion/verdigris/tarnish spots and/or streaking where the cooling system has leaked..
Maybe a small hose to a coolant reservoir.
Just a couple of thoughts.