Sounds like a head space problem. Do you know exactly where it drags? Like by the loading gate? You would have to put one round in and play with it a bit to see where it's draging. A dummy round would be best, or maybe a fired case if your worried about a live round. I'm wondering if it drags by the loading gate, it is possible the loading gate is loose, I have seem them get loose before, you would have to take it apart to tighten that up if that is the problem.
Should be some brass showing where it drags, take out the cylinder and look for that.
Let us know what you find out.