I had the same thing happen to a pre-15. I bought it at a gunshow and didn't shoot it for a couple of years. When I ran 6 rounds thru it it locked up. I went to a local gunshop that had a gunsmith and it looked thru his box of take offs and bought a used M-15 barrel. Took it home and unscrewed the old barrel after taking a punch and closing the split. screwed the new barrel in, put the pin in and checked the cylinder gap. the gap was a little close so I filed the back of the barrel until it was about 5 thou. the cylinder closed, the blue was about the same and I had all of $35.00 in the repair. The pistol shot great. I wouldn't be in a big hurry to repair it, look around at gunshows, I've seen M-15, 14 barrels that were new going for under $50.00