Mongo, it is perfectly legal to MAKE a firearm. However, if you want to make a sawed off shotgun, there is some paperwork you need to do first unless you want a new roommate named Bubba.
In the case of taking an 80% receiver, and building it into a rifle (or a pistol) that's fine- as long as a stocked gun has a rifled barrel at least 16 inches, or a smoothbore barrel at least 18 inches. A handgun may not have a smoothbore barrel if less than 18 inches.
Yes, you are thinking of the various .410 revolvers- they have rifled barrels.
PS- the legal term for a smoothbored pistol under the 1934 NFA is an Any Other Weapon.
And, over-all-length must be at least 26-inches.
Several years ago I bought a few Caspian 1911 receivers and built up a couple of pistols. During a compliance check, the agent told me that I was manufacturing and would need to apply for a manufacturers license if I was to continue building these pistols and "selling" same. The quirk in all of that is, according to this agent that, "if a person other than me, were to bring in a 1911 style receiver that he/she bought and I built it up into a custom pistol, that would be an OK process".
Now, whether that compliance agent was correct with his assessment, or not, I was NOT willing to apply for a manufacturing license at $1,000.00 for only a couple of 1911 builds per year.
Each and every time I get my FFL renewed another, updated book of rules and regs comes along. The first book I received in 1969 was around ½ inch thick. It will be interesting to see how thick the current volume is when it arrives shortly after my current renewal.
