Yeah, what to do when you run out of ammo for your futuristic-by-contemporary-standards guns would be a major problem for this type of story. I read the book years ago and don't really recall what Flint did about it, but he takes a pretty nuts-and-bolts procedural approach generally to how the characters cope with the lack of tech support/"there's no more where that came from" problem inherent in suddenly being plunked down in 1632 with only whatever supplies already happened to be in the geographical area that got transported. So there's probably some major story thread I've forgotten in which the chief twentieth-century badasses in charge assign somebody to start smelting ore and making bullet molds, etc., so they can eventually produce makeshift replacement ammunition for their massive pile of modern weapons.