Zip guns are the closest I can think of.
I disagree. Zip guns are just ingenuity used to create a cheap alternative to a more expensive metal propelling device.
To say it is perfectly legal to build your own firearm out of the extremes of either raw materials or an almost complete receiver on the market today, but then say it is illegal to do the same out of other suitable components is hypocritical.
That would be like saying it is illegal to make a knife from a leaf spring, or an old file, or a saw blade, or all the other things people use. But also saying it is perfectly legal to make the same knife from never before used sheet metal, as well as pieces of metal sold already shaped into a knife blank.
There is crude and unsafe designs as well as perfectly adequate ones, which use components that can withstand several times the operating pressure.
The only real difference in determining what is a "zip gun" is cosmetic.
If it looks like hardware components assembled into a gun (even if perfectly safe) then it is a "zip gun", but if has a nice fit and finish then it becomes a legal custom firearm.
Custom blued smooth bore barrel that can only withstand about 20,000PSI? Attached to a nice hand carved wooden stock? Assembled into a single shot break action shotgun? Legal.
Salvaged tubing that can withstand 100,000 PSI? Left looking like a piece of pipe, and attached to a crude looking stock? Zip gun.
The biggest irony is some "zip guns" are actually more modular than most traditional designs. And thus would be easier to customize to suite different needs or shooters.
So in America you can slap almost anything together you want, or build any tool from miscellaneous components. Just not a gun. A gun must be made from only gun components, even when inferior to a non-gun part. It must look like what a gun is expected to look like.
Yeah that sounds like freedom.