Ok, far as I know the rule on this is that they must fit the original gun. This is how M14 mags are used in the AR-10, and FAL mags in the MAS-49/56. But... What about modifying the capacity? What about taking a 30rd AK mag and splicing it in a way to produce a 40 or 50rd mag? Would you even have to splice two 30s, or could you just extend the one thirty? And what about building a drum off of one? Making drums for rifles that never had them, like the HK-91?