Wilson Combat Bolt Carrier features a drop in fit for any quality 5.56 case head based AR rifle. Precision machined from Carpenter 158 shot peened for increased strength and includes a tool steel extractor and ejector appropriate extractor spring and randomly batch HPT (High Pressure Tested). *Note- The only difference between “semi-auto” and “full-auto” carriers are the carrier weight and external profile. The “full-auto” carrier is slightly heavier to provide more mass and lower cyclic rate that can benefit carbine length and shorter gas systems short barreled/suppressed rifles and machine guns. The “semi-auto” carrier is slightly lighter to provide the least amount of recoil impulse for rifle length gas systems and its design makes reinstalling the firing pin easier after cleaning. Both carrier types are considered completely interchangeable.