Oh yeah... definitely a no-no... I got lucky when I didn't know any better and used 100 rounds through an FAL I had just bought... all it did was vary from shot to shot between throwing brass across 5 shooting lanes to not even ejecting the brass at all... and every distance in between.
I guess my first clue shoulda been that the gun shop I bought the ammo from had 4 bins of surplus .308. 3 of those bins were empty, and the 4th bin... Indian stuff, had plenty.