A conversion kit and method for converting the RUGER.RTM. 10/22.RTM. semi-automatic .22 caliber rim fire rifle to shoot .17 HM 2 cartridges utilizes the original bolt in the receiver and replaces the bolt operating handle with increased weight to replace the original bolt handle along with a .17 caliber barrel to permit the modified rifle to safely fire, eject and reload the .17 caliber cartridges.