When a cartridge does not fully chamber in a model 60, it will cause misfires and bulged cases just below the rim.
That's the same thoughts I had, but with a 39 the only way it could not fully chamber is if there were damage to the locking surfaces leaving excessive head space. I would try putting pieces of tape on a spent case (from something else that did not leave bulges) and see if you can still fully close the action. I would hope the gunsmith had already checked for that and it's not the problem because that could get expensive.
