is it because guns are easy to get without printing?
Because it is possible to print molds for cast iron, I wonder what else you need beyond that (although, don't indulge me if the topic is going in the illegal direction).
not a gunsmith, but cast iron manages to be both soft and brittle at the same time. and the barrel and bearing parts would have to be machined anyways. you have to try to harden it too. its probably easier to just machine the whole thing out of decent quality steel. just guessing.
Because it is possible to print molds for cast iron, I wonder what else you need beyond that (although, don't indulge me if the topic is going in the illegal direction).