The Constitution was never intended to spell out all laws of the country. It's a framework for how our government should work and a list of fundamental rights that should be protected at all costs.

The second amendment doesn't define "arms" because (a) at the time there wasn't much ambiguity there and (b) "arms" isn't actually the most important concept there. The second amendment enshrines the right for citizens to be able to stand up militias and defend themselves. The US didn't have a standing army until WW2, despite Alexander Hamilton's opinions on the matter. The second amendment was put in place because colonists lived under the thumb of a monarch and at the end of an army's barrel with nothing guaranteeing the people a right to defend themselves, their neighbors, or their fellow colonists (eventually countrymen).

The Constitution is a legal document and the foundation of all American law. It turns out a specific definition of "arms" would actually be very useful to the modern legal doctrines of the post-industrial society in which we actually live, as opposed to the pre-industrial agrarian society for which the British re-establishing their colonies, slave revolts and uprisings from Native Americans were problems worth worrying about.

The second amendment was a reaction to having lived under the oppression of British rule, not concerns over slave revolts or native uprisings.

That aside, the concept of amendments exist for a reason. It's totally reasonable for Congress today to amend the Constitution if a definition of "arms" is now needed. It wouldn't be the first time a new amendment modified or entirely voided an earlier amendment.

What we don't need is court rulings, executive actions, or even new legislation short of an amendment trying to modify or redefine an existing amendment. If an amendment needs to be changed or clarified that needs to happen at the level of another amendment, anything less is short cutting the system and, in my opinion, not democratic.