This 9mm BCG is perfect for your 9mm Pistol Caliber AR-15 Build. Due to the increased rearward velocity, blowback AR systems require a heavier carrier. This AR9 Colt pattern bolt comes in at 14.8 ounces, providing the correct weight needed to tame your blow back system. Compatible with both Colt and Glock pattern builds. This BCG works great with any Colt style 9mm rifle and Colt pattern magazines. We highly recommend ASC mags, which you can purchase here Colt & Glock Compatible USA made 8620 Steel Carrier Construction Phosphated or Nitrided Finish