steel cased ammo
The rim thickness on steel cased ammo can vary a lot.
Depending upon what kind of gun you're using, that can put a lot of pressure on the extractor claw and cause it to fail. Eventually.
I went through LFI Advanced Handgun Skills back in 2000 using a nearly new Sig 226 in 9mm. I shot Wolf steel cased ammo as an experiment. The bottom corner of the extractor cracked right about the time I finished the 1000 rnd case.
However, I know guys who have run lots of steel cased 9mm through various models of Glock without a problem.