Trivia: Some of the combat footage shown during the opening credits comes from John Huston's classic Civil War film, "The Red Badge of Courage" (1951). (00:08:22)
Trivia: Kyle Eastwood (Clint Eastwood's son) can be glimpsed very briefly in the opening sequence helping his father Josey work the land by their home.
Answer: At first, the story advances very rapidly, essentially giving the audience a primer lesson on Josey's angry motivation; so, many minute details aren't explored, such as where he acquired his ammunition. We might conjecture that Josey had a separate out-building, called a "powder house" (which was common in that era) where gunpowder and shot was kept for safety reasons. If he did, that raises the question of why the raiders didn't ransack and burn his powder house as well.
Charles Austin Miller