It HAS to be registered (has a serial number, basically) otherwise it's illegal, if you hand make a weapon out of complete scratch, and none of the parts are already registered (such as if you buy an AR lower receiver, it will already have a serial number on it and be registered to you as a weapon) then I believe you can take it to get it registered (not sure where) and a serial number will be put on it but I believe this is suppose to be done before assembling the weapon. As for making weapons such as fully automatic weapons, depends on your state. If full automatic weapons are illegal in your state, you cannot make a fully automatic firearm even if it's completely from scratch, same applies to other illegal weapons in your state. Also you need to be very careful in making firearms, always test it before without anyone holding it bc if something was done wrong it could literally blow up in your hand. And just some personally input, a shotgun shell firing revolver sounds like a bad idea, that would have a hell of a recoil. I know people who have had fingers ripped off by the recoil of firing a sawed off shotgun one handed.