Hour of Victory

Any headshots will work (you don't need the sniper rifle) and they are cumulative across the entire game. This is easiest to do with Bull while you are going for the sniper kills achievement. If you play on the easiest difficulty you are unlikely to die and can take all day to line up a headshot on the really stupid AI Nazis.

You might not even have to try to get this. You will unlock this achievement once you get your 10th headshot.