In 2013, Defense Distributed created the world's first 3D printed handgun, the .38-caliber Liberator. The following year, they unveiled an AR-15 receiver capable of firing hundreds of 5.56mm rounds without fail. This year, the team has been outdone by a group of fabricators at Printed Firearm who have once again raised the bar. They've successfully crafted and test fired the receiver for a Colt CM109 modular battle rifle -- the AR-15's badass big brother. The CM109 is larger and heavier than the AR-15 as it is built to accommodate a larger caliber round: the 7.62 x 51mm NATO. 7.62mm rounds fly farther and strike with much more force than the 5.56mm, making them far more deadly. It also means that the lower receiver (the bit that holds all the firearm's moving parts) has to be both heavier and sturdier to in order to handle the increased mechanical stresses and harder recoil associated with using a bigger bullet.