First make SURE that the rifle is marked .30-'06 and that the ammunition IS .30-'06. I know that sounds like I am insulting you, but ammo mixups have happened to a lot of very smart people.
For the following steps, I suggest you make up some dummy rounds without primer or powder. If you must use live rounds, find a safe area and be very careful!
I think it is a pretty safe bet that the chamber didn't shrink. Have you tried feeding rounds into the chamber with your hand without using the bolt? If the rounds go in OK that rules out anything having to do with the ammo.
If manually fed rounds go in OK, then try feeding rounds individually with the bolt (drop round in chamber and close bolt). If that works OK, your problem is feeding. Is your rifle the one with the rotary magazine or does it have the regular box magazine? The former was sometimes tricky and the spools are not interchangeable, so check to make sure the spool is marked .30-'06 as well.
After checking out this stuff, get back if the problem is still not solved.