Back to your original question, yes, you can shoot .308" projectiles in your rifle. You will have to use a smaller expander ball, but you are only talking about .001" all the way around, and a human hair is typically .004".
FWIW, the 7.62 X 39 die sets typically have two expander balls; one is specifically for shooting the .308" bullets, and a lot of guys do quite well with them. Just prep your brass first!
