The Colt 9mm rifle was originally designed to utilize the Uzi type magazines. High quality Uzi magazines properly modified work quite well. The back of the magazine must be relieved to clear the 9mm bolt catch or actuator. The magazine catch slot must be precisely placed to ensure reliable functioning. The front of the magazine should be stopped by the underside of the ramp and the feed lip will rest just below the ejector without impacting it. Use a milling machine with a 1/4" endmill to make all the cuts.