From what i've read and been told, these guns were made to have a working pressure of around 44000CUP, which translates to around 48ish thousand psi. That said, they're actually proofed to much higher than that. I read an article on some of the tests done during importing, and remember seeing something along the lines of them being able to handle upwards of 90kpst before having a catastrophic failure.

I've fired some IVI 81 headstamp 7.62 Nato in mine without any problems. The main problem with these is the surface hardened reciever has recesses for the recoil lugs, and these can become stretched from repeated firing of factory .308 Win. ammo.