I had a discussion with a few anti-gun lawyers once. The mental gymnastics required to interpret the 2nd Amendment in such a way is astounding. Basically, it boils down to the absurdity of denying what "keep and bear" means on the basis that those words are not "legally defined."
For instance, "possession" and "ownership" have very different meanings. They argue that "keep" is akin to "possession" and not "ownership," therefore the only arms people may keep are those specifically given to them or allowed by the state. Of course, within the context of the Bill of Rights, a document specifically written to limit the powers of government over people, rather than expand them, it's an absurd proposition, but it's important to understand someone's ideas before you can disprove them.