Mike,
An armorer friend of mine (who, sadly, passed away a couple of weeks ago) made a tapered bushing to fit the barrel taper at the appropriate spot. He would split that lengthwise, insert the barrel, and then dial it in with a 4-jaw chuck. As he reworked both M24 and various .50 calibre sniper's rifles for the U.S. military, I would assume that this was sufficiently accurate.