I tried a box (20 rounds) of the steel cased Russian ammo in my AR just to see if it would feed and do alright. I don't remember the particular brand, but if I am not mistaken, most Russian ammo is similar. It fed great and left no marks on the spent cases. After just the one box of 20 rounds there was a purple goo all over the bolt face and in the firing pin hole. It was from the red sealant they put on the primers. It took me a while to get it cleaned out but it didn't cause any trouble. This was only 20 rounds though. I could definitely imagine that after a range session of a couple hundred rounds having things get all gummed up from it. I have not shot any more of this ammo in my AR, as I said it was only a test. I stick with the brass cased ammo. I didn't really see any problems with it in my little experience and possibly you would only be stuck with some extra cleaning.