All materials have intrinsic limits. Anyone who has picked up a shell casing after a shot knows they can get very hot. The most basic 3-D printing materials are PLA and ABS, both of which melt and deform at high temperatures. I cannot imagine that a firearm made from these materials would last very long without losing effectiveness and function, and become dangerous to the user. Metal printing technology is extremely expensive right now so I'm not too concerned in the short term. That said, additive manufacturing will only get better and more affordable.
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