Steel cases, combined with dirty powders in Russian ammo. I’ve had this happen, and have seen this happen on several DI AR15, piston AR15 and a Tavor. Everyone has heard the whole thing about “if your rifle doesn’t run steel, then it’s a crap rifle”, but there’s a reason why you don’t hear it happen with brass nearly as often. Having to scrub the chamber every couple of hundred rounds is a waste of my time. Having to use a rod to knock out stuck cases with ripped off rims is a waste of my time. Only shooting a few rounds to prevent heat, to minimize the chances of a stuck round is a waste of my time. On top of that, Tula chronos at lower velocity and shoots less accurately than even the cheapest brass case ammo, which means that even if it runs in your gun, it’s only good for 50 yards and under training and plinking. Personally, I don’t use steel case ammo in .223/5.56 guns.
I did run Tula/Wolf/Barnaul in 7.62x39 guns, when I had those, but the more aggressive taper of that cartridge contributes to it’s extraction, and I wasn’t worried about hitting anything past 200 yards with any of them. I still use steel case 7.62x54R in bolt guns and 9mm in blowback PCCs.