The 5.7x28mm round is a whole boatload of fun, but sometimes, you just want an inexpensive plinker to enjoy at the range, right? Well, CMMG Inc. is making it easy to do just that, thanks to the company's Mk57 Conversion Kit that changes your 5.7x28mm upper receiver over to cheap, easy-to-get .22 LR! The simple design of the CMMG Mk57 Conversion Kit makes it fast and easy for you to change over from 5.7x28mm to .22 LR. All you have to do is take out the existing bolt-carrier group in your Mk57 upper receiver and replace it with this drop-in conversion unit. Once that's done, simply put your Mk57 upper receiver onto any mil-spec AR-15 receiver and load the included .22 LR conversion magazine. Bam! You're ready to shoot. No tools are required to switch over to .22 LR with this CMMG Mk57 conversion kit, and the included magazine fits all mil-spec AR-15 and Mk4 magazine wells.