FR-8s are built on M98-type large-ring Mauser actions, and have no problem with .308. Hundreds of thousands of M98 receivers have been built into .30-06, .270, .308, and other modern US calibers without incident. I would probably avoid Hornady Light Magnums and such, but standard hunting loads and such are just fine.
The FR-7s, on the other hand, were based on older, lower-pressure small-ring Mausers, and should NOT have been converted to .308. I would not want to be firing one of these pipe bombs.
AR15fan:
Advice I've seen: before trying to move the front site, soak with penetrating oil. I believe there may be a set screw also. Anyway, lots of folks strip out the fine threads due to the rust and gunk that's common on these guns by being heavy-handed.
FR8's are pretty cool rifles, though. They were built up to help train the Spanish army on the CETME, so it's designed to mimic the CETME in a number of ways, including sights, flash suppressor, and bayonet attachment.
-Troy