honestly if you can 3d print something you can make something almost as strong out of wood, it just takes more effort

one could also easily make a disposable mold for a low-melting-point metal alloy, those are much stronger than 3d prints and many can be melted on a normal stove

I think the problem is more that information on how to make guns is now easily available, rather than the specific usefulness of 3d printing as a manufacturing technique