What caliber are they? The US often made .30 caliber bullets with washed steel jackets when the price of copper was too high. These steel jacketed bullets look like regular FMJ's, but would attract a magnet. Most of what you would get from the CMP back when they were selling LC ammo was steel jacketed. I wouldn't be suprised if some .45 bullets weren't made the same way.
These washed steel jacketed bullets are perfectly fine for your rifle, and won't cause any more wear than regular copper jacketed bullets. Steel can be made hard or soft. In this case, the steel jacket was made to have a hardness consistant with copper.