To wit, as near as I can tell from reading the docs and discussions posted on the Internet, a pistol-grip shotgun sold as a pistol grip shotgun (that is, having never been a regular stocked shotgun, such as the Mossberg Cruisers) is listed on the 4473s as neither a long gun nor a handgun, but "other." Since it was never a long gun, it is (allegedly) not subject to the 18-inch barrel limitation; only the overall length measurement of 26 inches or more.
There has been buzz on the Internet that the ATF letters meant pistol-gripped shotguns were to be classified as "Destructive Devices," that is a firearm with a bore diameter of .50 inch that doesn't meet some sort of undefined, legally vague "sporting purposes" standard that no one has a clue about. I don't think that's the case at this point. The Striker/"Street Sweeper" shotguns were banned under such ATF rulings. The ATF also ruled that extensive use of certain firearms in the shooting sports didn't qualify as "sporting purposes," apparently since there were no penguins involved or other nitwit Catch-22 reasoning. The way the ruling was explained to me years ago by the feds was that so many guns were used in the shooting sports that use in the shooting sports couldn't be construed as as a "sporting purpose." For that matter, my "explainer" said with a straight face, neither could hunting, since any gun could be used for hunting various game. War is peace, and work shall set you free!
Street Sweeper Destructive Device
If, OTOH, you buy a shotgun with a stock (here's the GCA 1968 definition of a shotgun)and it comes with an accessory pistol grip, or you buy an accessory pistol grip for your shotgun, the gun is always a long gun and subject to both overall length and barrel restrictions when you put the pistol grip on.
If you put a regular stock on your pistol-gripped shotgun, then that's all hunky-dory unless the barrel length is less than 18 inches, in which case you have "manufactured" an SBS, a short-barreled shotgun, which you cannot legally do unless you fill out the appropriate NFA paperwork, meet the requirements, pay the $200 tax and wait for your tax stamp. Ditto if you hacksaw off the barrel of your regular shotgun to less than 18 inches, or you go less than 26-inch overall length — file the paperwork, pay the $200, wait for the tax stamp or get an all-expense paid vacation to the federal slam.
Here is the BATFE FAQs on SBS/SBR. Read it carefully!
You can change back to a non-controlled weapon without any heartburn, as per the FAQ, but only if you once again file the appropriate paperwork explaining the change with the BATFE. If you fail to file such a letter and you sell your now-Title 1 shogun to a friend, you may have sold her an NFA weapon, since the receiver, the part with the serial number, is the gun part of the gun. BATFE still has that receiver on their books as a controlled weapon, even though technically it isn't.
Ostensibly, a shotgun is specifically defined as having a smooth bore, e.g. no rifling. That means a pistol (such as the Ithaca Auto & Burglar double-barreled 20 gauge pistol) or revolver (such as the Colt New Service 44/40s with a smooth bore, designed for the Philippines I believe) having a smoothbore is a SBS and an NFA controlled weapon, unless that firearm has been specifically removed from the NFA, as has the Colt. Note that the Auto & Burglar has NOT been removed from the NFA!
If I were to take a hone to the barrel of one of my S&W 629s and grind out all the rifling so it would pattern better with .44 Mag shotshells...VIOLA! An SBS! If I hadn't filled out the paperwork, etc., I'd be guilty of a federal crime.
S&W Governor .410 Revolver
I could, OTOH, purchase a brand new S&W Governor revolver in .410 (and .45 Colt and .45 ACP) or any one of a zillion Taurus Judges and be happy as a clam, because the barreled is rifled, not smoothbored. Generally speaking, the rifling in these revolvers is "optimized" for shot, not the centerfire bullets. Some such guns, such as the Magnum Research BFR .410/.45, even though it has a rifled barrel, comes with a screw-in choke to allow it to pattern better with the .410. BTW, that choke is more like FULL than the listed Modified!
Serbu Super Shorty
A pistol made from a shotgun receiver that was never registered as a shotgun (e.g. stocked and with a barrel of 18 inches or longer) is an NFA firearm, but it is NOT a short-barreled shotgun, since it was never a shotgun. Instead, it is listed as Any Other Weapon, AOW, and it is subject to the same fed paperwork bot with a $5 instead of a $200 transfer tax. My Serbu Super Shorty, the coolest gun I own is an AOW. You can probably return an AOW to Title 1, but it requires the services of a White Wizard...
Rossi Ranch Hand
The mare's leg-styled Rossi Ranch Hand in .410, with a 12-inch barrel and an overall length [estimated] of roughly 24-inches is apparently an NFA weapon (as of last Friday) even though it has a rifled barrel. The Rossi Ranch Hand in .44 and .357 is a pistol even though it was made from a Winchester 1892 rifle receiver, because the receiver had never been a rifle...got that? If the Rossi Ranch Hand had a smooth bore at was at least 26 inches overall, my understanding is that it would be legal as a pistol-gripped shotgun, or as it's known on the 4473s, "other."
Taurus Raging Judge
The prototype 28-gauge Raging Judge is an NFA weapon because, well, because. Actually, probably because a 28-gauge shell is roughly .55 caliber, and even with a rifled barrel it falls into the DD category. A few years back the great Hamilton Bowen had to get an NFA Destructive Device tax stamp to build a Ruger Redhawk in .577 (lest Hamilton be set upon by feral Mac trucks!).
Wouldn't it be a hoot if today's ATF letter banned the importation of Chinese Winchester 1897 "Trench Gun" clones because they have bayonet lugs?!?!?!