Long first post, sorry in advance.
So I'm bored as hell at work and decided to do a little testing. Grabbed an old Colt BCG from the table, barrel, and a handfull of steel ammo. When I first built my rifle my buddy had said to buy a box or two from several different brands and tune mine to shoot anything. Changed the buffer weight once and since it has shot everything I try. So now I am in the spot of, do I spend the money on Brass or Steel? So I start testing a few things. The Hardness of this bolt face and extractor were from 77-79, My stainless barrel was 92, and the ammo ranged from 68-71. So in theory of metallurgy, the wear the steel ammo will put on the parts is negligible if any. So I look at the polymer coating they use and it appears the melting point on the poly is much higher than our rifles could get without being full auto, even then I don't see that being possible. Well that leaves me to think the only issue would be carbon build up from expansion/seal issues. I shoot about 100-250 rounds when I go to the range, I should be good there. So using average cost of both ammo, and expected life of a barrel( for us Navy/Marines it's 10,500 rounds) that's a savings of 840 dollars over the life of the barrel. I could build another rifle for that let alone the cost of just a barrel and extractor. So the final concern would be the very thin layer of copper on the projectile. Is it thin enough to withstand the rifling or not? For me the only way to find out is to shoot a shit ton of steel ammo while keeping the rifle clean and comparing to the amount of issues/longevity I have with my sons AR.