Obtaining Rare/Hidden weapons in RDR2?

I just want to start of by saying that I don't play the game, this is a question on behalf of my partner.

He finished the storyline a few weeks ago, played online a bit, got fed up and came back to single player. Now he's hoping to find some of the "hidden" weapons, specifically revolvers and other guns.

I've tried my best to see where and how to get these weapons, but most seem to come from completing missions in certain chapters. He has >!killed Micah!<and there are no more story missions if that helps describe at what point he is in the game?

Can someone please help me out with a list of weapons he'd be able to obtain at this point in the game? And if you would have to go about it in a different way than what's usually recommended in the guides?

Thanks in advance!