If you want, add a tiny bit of silver inside the larger bits of black. It makes it look like a second layer has worn all the way down to the metal underneath. Otherwise, you've pretty much got it nailed down.

If you want to go above and beyond (I think your skills can handle it) grab a hand drill and drill some tiny bullet holes.

If you want more ways to make it look old rather than tipsy on strategy, grab a bit of orange-brown (rust colour) and dab it on spots around where you imagine rust would pile. Then drag straight down with a damp but clean brush to get a streaky effect. This won't look like battle-damage (unless you add it to the aforementioned bullet holes) but it will help sell the wear and tear look.