7.62x54 is NOWHERE CLOSE and cannot be fired (ore even chambered) in a .308 or 7.62x51 gun. Whoever told you that is wrong.
Externally, the cartidge dimensions are the same or very close and the bullets are the same diameter.
SAAMI spec .308 ammunition allows for higher pressure cartridge ranges than the NATO spec pressure. So we have thinner brass, and potentially higher pressure. Fine in bolt guns with SAAMI spec chambers. Not so great in loose chambered, gas operated, semi guns.
I have personally seen a CETME fire Federal factory .308 WIN ammunition, and almost half of the fired brass was cracked immediately.
This CETME could have had excessive headspace (I dont know)... but you probably would not have seen these results shooting lower pressure, thicker brass, 7.62x51 NATO surplus ammo.
Dont confuse the .308/7.62x51 issue, with the .223/5.56x45 issue. While they both have distinctions - they differ.
To be safe - buy good quality 7.62x51 surplus ammo for your Cetme. This means Hirtenberger, Aussie, South African, Portugese, Lake City (quality is questionable), Argentine.
Stay away from Indian.
The South African is really the most available right now.... and the best deal:
www.aimsurplus.com/acatalog/copy_of__308.htmlAs a general rule - I dont shoot de-linked ammo, after seeing the problems that some of the Lake City had... and it was surpised that was because of the links contact areas weakening the case wall.