Classic arms and a few other places were selling czech made 7.62x54 uk59 heavy machine gun barrels that had been cut down and recountered as heavy bull barrels, I bought two sometime back for $69 each to use on future projects. I don't see any offered on their site now, if you google heavy mosin nagant barrel, you will get a lot of info. The other option would be to get a barrel blank turned and threaded, then fit and chamber it.