3D printed 9mm semi-auto

I would actually print it myself using PETG. It has similar properties to Poly-carbonate.

Well, in my experience printing extruder mounts in both PETG and ABS, I found that PETG fatigues and slowly bends to whatever stress there is (like the tension arm pulling on it). ABS holds its shape. I don't know what AR-15s are made of but I'd guess either nylon or ABS.

PLA softens at 50C and ABS softens t 90C (194F).  Putting a hundred rounds down the tube could easily bring the barrel beyond the melting point of any plastic, so the designer has to allow for thermal breaks, insulation or whatever OR limit the rate of fire so it does't overheat. Part of gun discipline is knowing not to overstress your weapon.