Wish everybody would post what gun they are shooting. From the sounds of it, it's a model issue. Yes the ammo is to blame but some guns are picky and others aren't.
I have the Su-16 and my friends has a norinco uppers and both shoot the ammo just fine. It's damn loud though!!!
I was shooting an LMT 10.5. The difference in the shoulder length is many times more than the depth of a firing pin indentation. I don't think it was a gun issue since I think any gun would not have been able to fire that cartridge given the huge out-of-spec dimensions. My LMT has been flawless through 4000+ rounds.