You can't copyright the functional parts of an item, and since most of the features of a gun are "functional" you generally can't copyright the design of a gun.

You can patent functional portions of a gun, assuming that what you're patenting is a new invention. But patents last 20 years at most so any patents that may have existed for the AK-47 or AR-15 have long since expired.

You also try to trademark a gun design, which is something that Glock is notable for having accomplished. But the short of it is that its very hard to do, and very easy to get around a design trademark like that. For example, companies are able to get around Glock's trademark on their gun design by just rounding the front of the gun a little bit. That change is so subtle that a casual observer wouldn't be able to tell the difference between a Glock and a legal knockoff.

The only thing that is easy to do is to trademark a gun name. For example, Colt owns the trademark on the name "AR-15". Because of that, only Colt can call its guns AR-15's. But again, that's easy to get around because a company can produce the exact same gun and just call it something else.