This is the Foxtrot Mike FM45 Mega Mass Lower Receiver . It is called the Mega Mass lower because it ships with a massive 13oz buffer in the gun to begin with. Most places will use a 6 or 7oz buffer just to make the gun run, but with blowback from the round most people end up adding a heaving buffer after the fact. Foxtrot decided to skip that step by just including the common buffer weight to begin with. This should result in a much smoother and softer recoiling build! Also includes an ambi safety. Immediate upgrades that will make any build you do a lot more fun to shoot. Designed to work with Glock 45 Caliber magazines and most .45ACP Glock parts.