I did a lot of searching through google for this, but the results were inconclusive. I used the terms "can bullets pierce plate armor" "strength of medeival armor agains bullets" and I've checked up on different types of guns in relation to armor.
The setting is a fantasty story in a sort of AU Earth, around the time that settlers were populating north america.
A lot of the information I found was about modern weapons vs. plate armor. Of the articles I found on early weapons, some said that even arrows could pierce plate armor, while others said they couldn't and that even early firearms couldn't.
The type of firearms I was thinking of were something around the Ross Rifle, but if there are earlier types of firearms that would be less effective, I'm able to change. There was some information suggesting muskets would be less effective.
In order to offer a defense from early firearms, how thick would plate armor need to be? Would it be possible for bullets to ricochet off of it?
Is it at least plausible that someone could avoid gunshot wounds by wearing a suit of armor?