You know, Indy, I think you're right, I think the cupro-nickel jacket was used until the M1 ball cartridge was adopted in 1926.
That would have been the first recorded change, as denoted by the M1 designation. I doubt that the military would have simply changed the jacket material and not have given it a new designation.
I'm pretty sure that the .303 British (again, I'll have to check my copy of Huon's Rifle and Machine Gun Cartridges when I get home) had LONG abandoned the cupro-nickel bullet jacket by that time.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.