Cupronickel jacketing became a problem when velocities exceeded 2100 fps or so. Read some of Townsend Whelen's old articles on this. This was a major problem for the 1903 up until right after WW1. Ordnance figured out that adding a small amount of tin to the jacket alloy solved the metal fouling issue. The Germans had apparently stumbled on the same solution - perhaps never had the problem - as they had tin mixed in their priming compound from the earliest adoption of the 8mm Mauser.