I had a similar issue with a similar vintage model 70 and discovered a ring-shaped part of a cartridge neck had broken off and was still sitting in the neck of the chamber. I couldn't see it until I put a scope in the barrel, but ubsequent rounds would not go into the chamber fully, held back about 1/8" by the little ring of brass sitting in the chamber neck.
Good luck with it and tell us what you find!
Justin
