Don't currently own a fire arm but enjoy shooting. Have been hunting and expect to again in the future, though not anytime soon. Amusement with "Stupidity with Firearms" does not equate with hating or even disliking firearms, unless your real name is Wayne LaPierre.
Too bad, already limited, as the link says the ATF has determined them to be "Any Other Weapon" requiring approval and registration under the NFA.
I read into the ATF Report that it had to be with the flare pistol.
Seems it's just the right size to toss in the tool box, thus exempting it from the NFA regulations.
Kinda like having to have your ammo not loaded in the gun your transporting.
Little known fact. A flare gun cartridge does not contain gunpowder. Its actually a "rocket" much like fireworks. The shotgun primer is used to ignite the rocket "engine". Still, I think it would be safe to use an adaptor IF it was chambered for 22 rimfire.