Plastic 3D printed lower with metal parts for completion works well with a 22 cal AR-15 upper. When 3D printers that uses less brittle and bit more rubbery plastics are available for home use, printed AR-15 lowers would be more resilient to handle 5.56mm or .223 cal. Right now one can with a CAD program thicken the stress points of the original AR-15 lower without interfering with the moving parts and upper functions is possible to make the design more resilient with ABS. Problem with plastic is the heat generated from excessive and sustain firing can melt the lower plastic receiver. I do not know how well these ABS plastics can survive cold storage without it shattering under stress. Metal sinter 3D printers are available but very expensive to own privately. IMHO 22 cal AR-15 is possible with a 3 D printed lower. Anything heavier would have problems.
The original gears I redesigned were ABS. It’s not very strong, and UV light severely weakens it’s mechanical properties.
The production gears we used were made from PBT, which is about 4X stronger than ABS.