The arming was (and remains) mainly about keeping the US settler garrison empowered to carry out indigenous eviction, and keeping non-white populations under control. Not protecting from a tyrannical government.

You can see that when those groups did try to arm themselves, such as the Black panthers or the American Indian Movement, laws were passed and repression carried out to disarm them.

Highly recommend reading Roxanne Dunbar-Ortiz - Loaded, the disarming history of the second amendment for more on this. You can find the audiobook on torrents.

Can you legally shoot a cop if they were shooting at you for no reason?

That was a lie. The ones who don't like tyranny don't like guns, and the ones who like guns don't actually mind tyranny, so long as it hurts the right people.

This could cause a civil war.