It wasn't for the parts that go boom, it was custom ergonomic parts for a legally purchased, factory made firearm - the bits of the gun that you hold. Those are under less stress, so with the right filament and infill settings it holds up well enough. And when it starts to crack I can print another for $1 instead of buying one for $30-60. Plus I can change the shape and size to fit me better than an off the shelf part.

I had a buddy in high school who did competitive shooting, he did the same thing by carving these parts from wood, now it can be done for a fraction if the cost. Check out Olympic shooting pictures for an idea of the parts I'm talking about.