When a can doesn't have a can

[Malice4you 627]

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

[BlueLineFish 615]

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