Incorrect. There were a variety of multi-shot and/or breech loading firearms available.

That's also not how Constitutional rights are interpreted. It's intended to widely protect citizens from government interferece. That's why the 2nd Amendment says "arms" and not "muskets." It applies to things like knives and swords as well as firearms and will also apply to laser guns and other scifi technology.

Apply this "snapshot" theory to literally any other Constitutional right if it still doesn't make sense.

First Amendment applies to every electronic form of communication. There was no internet, TV, radio, and so on. Does that mean the first amendment doesn't apply? Because it does.

Do search and seizure protections apply to wiretaps? Those certainly didn't exist when written.

Third Amendment says "house" so does it not apply to apartments, dorms, condos, hotels, lofts, and so on?

How about the Seventh? $20 was worth far more back then, does that still apply? Why?

Muskets are irrelevant. Technological development is irrelevant. The Second Amendment protects the right of the people to keep and bear arms. First Amendment protects speech over electronic media. Fourth Amendment applies to electronic surveillance.