The FR-8 is chambered for 7.62 X 51 NATO NOT .308. Will it blow up from one round of .308? Probably not. Remember, the receiver and bolt are 70 years old or so. They were used for some time in an 8mm Mauser chambered rifle. The current barrel was screwed in in the mid 50's and was shot some more. If you are willing to risk your face under these circumstances, go for it.
I shoot primarily reduced charge handloads in my FR-8. Not extremely reduced, just about 2 grains under max. I stick to 147/150 gr bullets. Going heavier puts additional strain on the gun.
For SHTF, you would be best served avoiding "Light Magnum", "Superformance" or any ammo listed as "High Performance". Also stick to the light to mid-weight bullets (Under 150) to keep bolt thrust manageable.
Anyone who tells you any .308 ammo is good to go is either stupid or has designs on your wife/girlfriend.