Well, this is a long shot Grouse, but you might give it a try.

Green Mountain makes a 15/16", 45 caliber pistol barrel in a 1:20 twist - but they are only 12" long. You might try calling them to see if they can produce a rifle length barrel for you. If so, they can probably even install your existing breech plug if you pull it off and send it to them. Then you can drill a touch hole where you want it, or install a liner.

http://www.gmriflebarrel.com/productdetail.aspx?id=323872