U.S.A.–-(Ammoland.com)- The .223 Remington cartridge is a high-intensity rifle and pistol cartridge. The SAAMI (Sporting Arms and Ammunition Manufacturers' Institute, Inc) allowable pressure level for the .223 is 55,000 psi.
In most “3D printed” AR-15 type rifles, only the lower receivers are 3D printed for the platforms they are on. The lower receiver holds together the fire control system. There is little mechanical strength needed. Upper receivers have not generally been printed because they are easily available in the United States. They are considered a firearm part, like a buttstock, a bolt, sights, or a magazine. They can be purchased over the counter or through the mail as part of ordinary commerce.
The rifle pictured in the Twitter post above is different. It is not using an AR-15 lower or an AR-15 upper. It is labeled a “bolt action”, that is a manually operated rifle.