I'd allow exceptional melee weaons to be raised one weapon rating via Craftsmanship but not Guns. My reasoning is simple: a sword's weapon rating comes from the sword itself, how sharp, well balanced, and so on it is, but a gun's comes from the bullets you load in it.
Now, it would make some logical sense to have exceptional guns add +1 to the Guns skill of anyone using them (since they aim so much better and so on), but that's a mechanical minefield that I wouldn't want to open up. Ditto the Items of Quality mentioned above.
Personally, I'd handle it with a high rating in Craftsmanship, the Aspect mentioned above, and a Stunt ('My Own Guns') granting him +1 whenever he's using a gun of his own design.
You can also do things like make a gun pretty much untraceable or fully automatic if you've got that kind of skill, and those are both really nice in and of themselves.
He could also likely make armor-piercing ammo (which I'd say ignores a point or two of armor), a normally illegal ammunition type.