TNT, You ought to look at www.weaponsguild.com. Thats a kit building site, and they are the people who first successfully modified a standard thompson smg kit into a legal s/a thompson. I think trying to modify that KAHR into a good shooting gun will be an exercise in futility. But, to answer your questions, try greenmountain rifle barrels. They make an exact copy of the TSMG 1928 barrel, and they are only about $125.