If old, the jackets might be cupro-nickel, a common jacket material through the 1930's. If later, they may be mild steel plated with cupro-nickel to keep the steel from rusting.
U.S. makers have not used cupro-nickel since about 1935, generally using gilding metal, which looks like copper. WWII U.S. bullets were usually steel jacketed, but plated with gilding metal rather than cupro-nickel for rust prevention.
The mild steel used in bullet jackets will not decrease barrel life unless you plan to fire an awful lot of it in short periods, like 750 rounds per minute.
Cupro-nickel jackets will cause a build up in barrel, but cupro-nickel plated steel will be no problem (check with a magnet).
Jim