The problem is that designing a gun isn't much harder than reloading your own ammo (Samuel Colt designed his first gun at the age of like 16). All you need is a shop and a basic understanding of the physics and mechanics involved and you can make a working firearm. Of course, tighter tolerances are always better, but it isn't some kind of arcane art
Exactly. If you can machine your own ammo down to working tolerances, you can make your own gun too, no 3D printer needed. It's just a different level of skill there.
The people who can make their own guns and ammo aren't who we're talking about. There will come a time soon when people without any knowledge or skill will simply be able to download a 3D plan file and print a working gun, then buy the ammo at wal-mart. At that point, the gun control argument is moot, it doesn't matter whether you're for or against gun control. So we should be arguing over bullet control, not gun control.