Mild steel has been used as bullet jacket material for over 100 years.
It's a bit tougher on barrels than guilding metal or cupro nickel. Germany used it extensively during World War II, especially later in the war as copper became ever scarcer.
Cupronickel is an older style jacket material that was used extensively in US military rifle ammunition until the adoption of the Ball M1 cartridge in the 1920s, which introduced the guilding metal jacket.
Cupronickel jacketed bullets had a nasty tendency to badly foul the bore, requiring laborious cleaning and often the use of really nasty chemicals that could quickly destroy the bore, like heavy duty (28% or more) ammonia or combinations of hydrogen peroxide and vinegar.
I'm not sure when the military stopped using cupronickel to jacket handgun ammo, but I know that the fouling problems weren't as extensive in the .45.
"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.