The “fire in a crowded theater” argument is pretty sound, in my opinion. The right to free speech in the 1st amendment is pretty unequivocal, but there are exceptions that have been created in the law for perfectly valid reasons (slander is another example), and these do not negatively impact free speech in any significant way. Clearly the right to free speech doesn’t just involve saying whatever words you like, but the specific context and meaning; nevertheless, these restrictions stand, for good reasons.
I fully agree with the right of citizens to bear arms, and as the amendment provides, if a state maintains a militia or decides to in the future, they can call up citizens with proper firearm training/experience to join. (A couple of asides: first, a militia in this context is decidedly not a bunch of survivalist idiots shooting tin cans on the weekend and plotting the overthrow of the “fedrul guvmint”. In fact, even when organized by the states, Art 1 Sec 8 gives the federal government the right to suppress rebellions (See: U.S. Civil War). Second, most of the Supreme Court cases dealing with the 2nd amendment actually ruled against conscientious objectors who refused state militia service. If state militia service were to make a comeback, I would definitely want to see conscientious objectors get more respect.)
The Supreme Court ruled in Miller v. U.S. that the federal government could restrict certain arms (in this case, sawed-off shotguns) that were deemed not suitable for militia service. It’s arguable that AR-15s would be suitable (bump stocks less so), but I think it’s fairly clear that there are arms that do not belong in private hands. Would strong 2nd amendment advocates be ok with nuclear weapons in private hands? Or is it ok only if it looks like a gun? Because clearly the technology of guns has changed. The Las Vegas shooter was able to create such carnage in such a short amount of time that would have been unimaginable with the firearms available in the 1790s. I don’t believe the 2nd amendment disallows certain arms being restricted (fully automatic rifles are already banned, of course. Again, is that a violation of the 2nd amendment?), and certainly wouldn’t disallow universal background checks or other regulations that would be consistent with organizing a state militia.