99 .308
I think you posted this question some where else in this forum. Could have been someone else. Anyway, it would be a real mess to convert over. The older frame is plenty strong enough, but the internals are a little different. I'm not saying it can't be done, but if you try to load a .308 into it, you will see it won't go. I have never been asked to change one over because of the feeding problems involved. I never even bothered to look into it, because of that. It would probably cost a lot more than you would get out of it. Best to look for a beat up .243 to convert, or hold out till you find a .308.
Yes, the newer receivers spring also. A lot of guys that reloaded, and some with factory loads, use to complain to me about sticky extraction. Only thing to do is back off on your reloads. I owned a .243 that extracted hard with winchester and Federal ammo. I went to Remington and it fixed that. The other ammo was too hot, and less accurate.