My (first) argument is not an originalist or purposivist one, but rather a purely textual one. The primary meaning of “arms” is plural. Nick argues the plural is used to go with “the right of the people.” The real “reason,” I think, the plural term is used is probably because that is how it was written in the English Bill of Rights (and the Magna Carta). The question is what are the consequences of those possibly unconscious decisions and associations for a textual reading of the Constitution.
Certainly the plural arms goes with the plural “people.” But both are independent drafting choices. For example, the right of the people could have been “to be armed,” which would leave out the plural. Or it need not have been written in terms of “the People.” Nick compares it the Fourth Amendment. I like that: is the “people’s” right to be secure in their “houses, papers, and effects” even arguably singular, or be restricted to one house, one paper, one effect? Could papers be limited to one piece of paper? It is not “people” that makes “papers” plural, it is the way people commonly use paper.
Turning to purpose, the Framers used a plural word; they certainly did not intend to rule out “one gun” rules, because as far as I know, they had never encountered such restrictions, and were more interested in gun minimimums than maximums. None the less, the plural has consequences. Nor are the consequences absurd (this still permits two-gun limits) though they may be undesirable from certain policy perspectives. Nor is the reading contradicted by substantial originalist evidence. Thus there is little reason to depart from the plain (plural) language, even though it could be an “accident.”
Indeed, arms and papers have a lot in common. The plural form is the primary one specifically because they quite commonly are used in multiple units.