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I've printed this in PLA and ABS, experimenting with different # of shells, different levels of infill from 20% to 100%.
All results are the same. If you print this standing straight up as pictured, you must ride the AR-15's bolt closed onto the flag or it will break off inside the chamber and you'll have to rod the broken plug out of your barrel. Hit the bolt release and let the recoil spring ram the bolt home, and SNAP - every time, in almost exactly the same spot on the part. See attached photo.
The part is weakest in the Z axis direction, due to the nature of laying down the layers. I'm going to try printing it lying down on its side instead of standing straight up.