I read an article on steel cased ammo vs brass in AR15, I know we are talking about m14/305, anyways the final conclusion was that steel cased bimetal ammo reduced the barrel life by 50%. The barrels needed to be replaced after 6000-8000 rounds, where others still were shooting within specs after 20k. I can't remember the exact numbers but the difference was huge.
According to that article the copper washed steel ammo is just that: steel with a microscopic thin coating of copper which gets rubbed off on the lands of the rifling. The steel jacket of the bullet itself (not just the casing is steel) will wear out your barrel 50% faster.
However, the article did state that the amount you would save on ammo would allow you to buy a replacement barrel when it was worn out (on the AR15).
The one issue with the testing in the article was that they pumped 10,000 rounds of ammo through the barrels in a very short period of time, allowing the barrels to get very hot and possibly wearing them out faster.
Bottom line: if a magnet will stick to the bullet, it has a steel jacket




















































