Years ago, there was a "compensator" put out by Ares called something like the dragon's breath. It looked like it was 4 or 5" long, and for all purposes, was the innards of a can, just without the cylinder covering it.  I think there was a Sig PCC or rifle that had a similar short barrel with 5" "compensator", and I thought I'd read that the ATF stepped in and was doing a review before allowing/denying sales.

What is the legality of such a thing if:

Used as a compensator and no one is dumb enough to get a conveniently 5" long section of 1.5" pipe?

And

Permanently attached to a 11" barrel per ATF rules to make it 16+"?

 

And before I am asked, I have no interest in getting one for myself*, but a search on ebay for an unrelated item returned a result similar to the above and it got me curious.

 

*OK maybe the tiniest of interest of shooting something like that just once, but not actually ever buying one

The Ares breath muzzle brake. It had nubs on the outside to keep from wrapping a sleeve around it to make a suppressor. It was deemed legal