I'm lazy today, so I'll just copy & paste from one of my earlier posts on TFL:
There's a lot of folklore about the FR-8 out there.
The FR-8 is built on a large-ring M1943 Mauser action mated to a CETME-C barrel, and it's fine to shoot with commercial .308 Winchester or surplus 7.62x51mm NATO ammo.
The Spanish Army converted their M1916 small-ring Mausers into FR-7s, and their M1943 large-ring Mausers into FR-8s. This conversion was done in 1965, when Spain had already moved away from the 7.62x51mm CETME round to the full-power 7.62x51mm NATO. The CETME-C was introduced in 1964, and it was designed from the ground up to shoot nothing but 7.62mm NATO. Both the FR-7 and FR-8 use CETME-C barrels. The FR-8 is built on one of the strongest bolt action systems ever made, the large-ring Mauser, and those things shot 8mm Mauser before they were turned into FR-8s.
The Spanish Army used their new CETME rifles for a decade or so with 7.62mm CETME prior to the CETME-C introduction in 1964, but this was not done because the CETME rifles could not hold up to the higher pressure of 7.62mm NATO. The 7.62mm CETME was just a way to increase controlability of the CETME in full-auto fire.
(In the late 1950s and early 1960s, Spain sold CETME barrels and other parts to Germany for use in the HK G3, and those puppies never shot anything but full-power 7.62mm NATO.)
The Spaniards abandoned the lower-powered CETME round for logistical reasons, that's all.