(With my best Capt. Renault impression).
So, the ATF went and sent a letter (PDF link) to the manufacturer of Can Cannons that said their product was not a firearm, but that attaching it to a rifle receiver made the rifle a Short-Barreled-Rifle, or attaching it to a pistol receiver made the pistol an AOW. Cue Internet Rage(TM).
As you might have gathered, I’m not at all surprised by this determination. This device does not, after all, stop the receiver from being a “firearm,” being basically a variation on a blank-firing-adapter. And it’s shorter than 16″ (short-barreled), and a smoothbore (AOW for pistol). This is a consequence of the Can Cannon being a accessory to what is legally a firearm. If they had instead built it as a complete unit, completely unable to fire conventional ammo, things might be different. As it is, though, an AR-15 doesn’t stop being a firearm when you mount a Can Cannon on it.
This is just another reminder of how mind-numbingly stupid the patch to NFA’34 to allow handguns, but still effectively ban anything that could be considered a handgun but isn’t actually a handgun. (That sentence hurt just as much to write as it does to read, I assure you, but was the only way I could express how I understand the history of the Act.)
On the other hand, I am a little surprised they didn’t just rule the assembled weapon as a Destructive Device (that bore is way bigger that 0.50″)
EDIT: The ATF Determination Letter