Well you can make a cannon as easily, but the BATF has been aggressive at pursuing people who discuss gun making, the more sophisticated (like 3D printing them) the more aggressive the pursuit. A project for recreating a black powder musket? Not a lot of heat. A project for a repeating launcher that lobs soviet era grenades while pulling the pins on exit, very aggressive pursuit.
There's plans for an 7.62 Tokarev pistol based on the AK style action that have been floating around the internet since forever. No drill press or router required. Just a drill, file and some misc hand tools. The AK style action lends itself well to that kind of ghetto fabrication because it's a stamped receiver and machining tolerances can be described as "optional"
It's legal to build your own guns but people who disseminate info for how to do so have got in all kinds of legal trouble. Defense Distributed, codeisfreespeech.com, etc
