Had the same problem with a 1913 Sterlingworth. The gun had to be forced the last bit to close. It would close fine on original paper shells from the 1930's but wouldn't close on modern shells, especially Winchesters. The rim thickness and DIAMETER was larger on the modern shells. Paid a good gunsmith to recut the rim recess to modern specs but left the chamber length the original 2 5/8". Gun closes fine now. I think this may be your problem.