Quoted By:
Here's a little quirk of Federal Laws on firearms in the United States. The 1934 National Firearms Act states that a shotgun with a barrel shorted down to shorter than 18" or an overall length shorter than 26" is a 'Short Barreled Shotgun' and subject to a $200 tax and registration (which was an absolute shitload of money in 1934, mind), creating one without first having your tax stamp ready, or being something like a Type 07 Special Occupation Taxpayer, is a crime that can lead up to 10 years in jail.
Here's a variant of the Mossberg 500, sometimes referred to as a 'Shockwave'
As you can see, it's quite short, yet it's not subject to the NFA, why is this? Mossberg manufactured this gun, and did not declare it to be Shotgun, which it isn't because it lacks a stock, and it's not an Any Other Weapon, the category which things like Smoothbore Pistols tends to get lumped into, and some short shotguns, because it's over 26" in total length.
It never had a stock from the factory, so it's not a Shotgun or Short Barreled Shotgun, it's too long to be considered a Smoothbore Pistol, so the Bureau Of Alcohol Tobacco, Firerms & Explosives, BATFE, or ATF for short (their older acronym), have written themselves into a corner and just refers to guns like these as "Firearms." Legally speaking, a "Firearm" in this context is a category of guns which aren't rifles, pistols, or shotguns, and also doesn't get into any of the subjects restricted by the NFA.
If you took a regular Mossberg 500 and then cut down the stock and barrel to this length, that would be illegal without the relevant paperwork and taxes, but if Mossberg puts it together like this at the factory, and it never had a shoulder stock attached, it gets to be in this goofy little gray area. If you put a stock on it, that's not ok legally, nor is it okay to put a regular pistolgrip on it unless you somehow make the gun overall 26" long.
