The problem with firearms is of course the "thought crime" aspect of the recent/current? BATF stance on making a firearm w/o an FFL, using it, and selling it, as well as ITAR concerns.

Naturally, there are DefCAD/Defense Distributed and the like and a couple of very active subreddits/forums and some new developments/techniques.

Arguably, 1st Amendment + 2nd Amendment == the right to 3D print firearms, but of course, no one aside from Cody Wilson wants to be the poster child for that court case, note the example made of Philip Luty in England and his writings.

For internal combustion engines, small models are popular things (have you at least purchased a Revell Visible V8 model?)