Has anyone used this lightweight spanish 7.62x51 ball ammo? It throws a 112 grain FMJ and is supposed to be designed for the CETME, so theoretically is should work well in my HK91 clone. Has anyone used any of this stuff? Is it reliable?
The lightweight Spanish ammo (112gr) was purpose built for use in specially modified light machine guns (lighter bolt and springs) and to allow more "controllable" automatic fire from the ORIGINAL Spanish CETME design rifles (the prototype "A" and "B" rifles specifically). Once HK got involved in the project and the G3 was born, a whole completely different set of specifications were brought to bear, including use of full power NATO spec ammo (Germans are big burly men...don't want or need no sissy/ lightweight ammo for controlled FA fire). It is unlikely that it will cycle correctly in an HK91 clone or in any similar rifle without modifications that you aren't going to want to make (because of both cost and having to hack up your rifle). Furthermore, if memory serves (and its been a while since I have seen any of this stuff so don't quote me on this) that ammo is corrosive which you need to be cautious of as well.
I'd skip it and find something of more recent vintage and of better quality that will function correctly in your rifle.