I purchased a 20" AR upper in 7.62x39 from Model1sales a few years ago.
I ran about 500 rnds of Wolf steel cased ammo through it. The casings had some sort of lacquer coating. If I shot more than about 50-100 rounds through it, it would start to jam (shells getting stuck in the chamber and having to be forcefully removed). I think it was just the lacquer coating that would rub off and start causing things to bind up. After cleaning it would cycle fine again.
I reload and have since transitioned to mostly brass cased reloads for it. I'm not against shooting steel cased through mine again if I need to.
With wolf ammo I'd regularly put 10 rounds into a 1" hole at about 75 yds. With reloads I seem to get the same.
I think i have about 700 rounds total through the gun now. I haven't noticed any problems with the bolt, chamber, or barrel. I haven't done any close inspections of them, but the gun still shoots fine and I haven't had reason to suspect any damage to it due to steel cased ammo.
It's not very scientific, but that's my experience.