Thats preposterous, we don't allow the 1st amendment to only apply to printing presses or hand made leaflets/books. The founders were also aware of things like the puckle gun and multi-barreled weapons systems, all of which could be owned by civilians.

There are a lot of reasons why you'd need a standard capacity magazine but the most important reason is "its not the business of the government to unconstitutionally deprive its citizens of their rights".