They could probably get their hands on A Henry Rifle or Sharps Carbine, those pre-date Colt's Peacemaker. Break open shotguns wouldn't be to hard to get either.
IIRC, the first commercial revovler to use metal cartridges was the Smith and Wesson Model 1(1857).

"Everyone's always in favor of saving Hitler's brain. But when you put it in the body of a great white shark, ooohh! Suddenly you've gone too far!" - Professor Hubert J. Farnsworth