Plastics with heat or fragility issues alone. One plastic might not be able to handle heat very well or not at all. Another plastic might be too fragile to use. Finding one that can do well in both categories is tricky enough already for these two issues alone and even then it's usually not above 250 C. PLA plastic alone melts around 200 C. PLA - glass transition is even lower than that at 60 C or 65 C (roughly 150F) which would be a no go on heat alone to me personally for any type of rifle at least if not other ones to as well.
This is an extreme video since he's not shooting in a real world manner for the most part. Doing his own personal weapon test with an AR-15, but for now to show the gun heating up and after 5 or so clips it catches on fire.
It catches on fire around 2:40 or so.
https://tube.poal.co/watch?v=Kzfm4pYhIyY
I think shooting 1 or 2 clips fast out of a 3D printed plastic one could easily warp the plastic if not melt it depending on what plastic is used for it. Plastic isn't going to be able to handle this kind of stuff in the video.
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