There are no rules, because what "a superb handgun" means is determined by yourself, or rather your group.
One group might decide that a superb handgun is a very accurate gun, so they give you a +1 on your guns roll with it. Others might decide it means the gun is deadlier and is now a weapon:3 instead of a weapon:2. And yet another group might do both.
Or "superb" is just an aspect to the gun, that might allow you to spend a fate point on it when you use the gun.
Then there is the fate fractal, where you can treat anything as if it is a character. So a "superb handgun" could be a character of its very own, with aspects and stunts and the like. As a character, the gun would get it's own aspects, skills and stunts and even an action in a conflict. The gun could make a maneuver for the shooter to tag on his attack, for example.
Or you just use the craftsmanship as an excuse to take a stunt that lets you use you craftsmanship to do guns maneuvers. Or let your craftsmanship compliment your guns roll. Or anything you like.
The same goes for the number of shifts required for such a roll. Or even if you roll at all. Or how you roll. If you want the creation of a gun be something really epic, you could make it a conflict. The craftsman would "fight" against the concept he has of the new weapon, and the concept's complexity (how complicated it is to build) would fight back. A consequence for the craftsman could mean that he has been working and working without rest, and it's taking its toll. One exchange could be a day, a week, a month, depending on how long you think the creation of such a weapon would take. If he can take out the concept in this fight, he was able to create the weapon.
The thing with +1 for sticky aspects is under the magic rules. For mundane aspects, you just roll, sometimes against a fixed number, sometimes against an opponent, and if you tie, the aspect is fragile, if you have at least one shift over the target, the aspect is sticky.
There was a rule of thumb somewhere that you start at +3 for maneuvers and then add 2 for every factor that would be boring or subtract 2 for any factor that would make it more awesome.
That's just how fate rolls. There are a ton of different ways to deal with the exact same situation. That's why I said that it highly depends on what you actually want to do.