I’ve read all Hatcher’s information regarding the bore metal fouling propensities of cupronickel bullet jackets in the .30-'06 up until the early 1920’s when GM jackets took over. My understanding is that the use of CN jackets in the early days generally did not cause a serious metal fouling problem in the .30-40 Krag, or the .30-'03, as velocities were substantially lower than the .30-'06, but that firing more than a few rounds of the higher-velocity CN-jacketed .30-'06 would result in heavy fouling and rapid loss of accuracy.
What I have never read anything about was combat performance problems of CN-jacketed ammunition used in the WWI '03 Springfields and 1917 Enfields. I’d think in combat, cleaning out metal fouling from rifle bores in the trenches would have been well-nigh impossible, and accuracy would have been nonexistent, except at close range. Granted, there were very few US-made automatic weapons used in the trenches of WWI, but I would think that CN fouling problems would have been magnified greatly in them. So how did the Doughboys cope? Or did anyone even worry about it?