Great episode tonight, I was really surprised by this. The guy who lost his daughter not so much, but it was good seeing the characters together and we even got to see more backstory stuff.
I'm not sure if the producers ever said they consulted experts on this. But without more information about how to make bullets without modern machinery, then I still buy the muskets usage.
At the very least, maybe Monroe is hoarding the modern bullets and weaponry and letting his soldiers use the more basic and crude stuff.