Well, the only ammo would be your fists or what you could fashion out of… um… nearby rocks.

The ammo went over the waterfall.

And the indigenous species would all be rabbits or prairie dogs, and somebody gets infected when they’re fighting a buffalo (the buffalo got away) and they die of gangrene and then someone gets bitten by a rattlesnake and shoots the only other remaining guy because he’s hallucinating from the venom, and the other guy lives but he dies a death of dysentery and starvation over a matter of weeks.