I took a buddy of mine to the range with a barely used mint PTR91 I found for him on the AR15com forum. The only ammo we took was Israeli IMI 7.62, but the rifle would not fire any of it. It didn't appear that the firing pin was even striking the primer. I took it apart and the firing pin doesn't appear damaged, still round on the tip. We tried several times but no luck, the former owner never had a problem with it. Any ideas?
Will it fire other ammo?
Do you have another .308 rifle you could shoot that ammo in?
There is a whole littany of things which could cause this -
It could be in spec but bad ammo (primers got oil in them, whatever), or it could be out of spec ammo and preventing the bolt head from fully locking.
The rifle could need a good cleaning, also preventing the bolt head from fully locking.
The hammer spring could be worn, and it may have shot different ammo, but not surplus military stuff which frequently has harder primers.
Maybe you loaded the rounds into the magazine in the wrong direction.

Kidding, only kidding.