I see that Dragline45 edited his original post, so my reply may be in regards to something that's now out of context. But, just in case there's some confusion over the muzzle device legality....
Mmmm, are you sure about that?
If it starts out as a rifle, and you cut the barrel back past 16", its now a SBR, is it not?
I believe the Mares Legs are sold as handguns.
If you permanently install a muzzle device that brings the barrel length back to at least 16 inches, it is legal. The "permanently installed" qualifier is the key. Permanently installing something makes it integral to the barrel and, therefore, a
part of the barrel - not an attachment.
I own an upper receiver for an AR-style rifle, which has a 14.5" barrel. However, it also has a pinned and welded muzzle brake installed. That brake brings (legal) barrel length to 16.25". As such, it can be a "rifle" without the need for a tax stamp. It doesn't change the fact that performance is based on the muzzle being at the 14.5" mark, but it does make it
legal as a rifle.
On the other hand... when I was initially testing that upper receiver and had not yet had the muzzle brake permanently installed, it was 100% illegal to install on a lower receiver with a butt stock. For testing, I had to use that beast on a "pistol" lower, in order to make sure I wouldn't get a complimentary visit from the ATF.
I could even make the upper receiver for my 7.5" AR pistol into a "rifle" upper, if I wanted. All I would need to do is have a 10" piece of tubing threaded, and then get it pinned and welded for permanent installation. At that point, it would be a 17.5" barrel, in the eyes of the law.
It would be stupid, because I would have the performance of a 7.5" barrel, combined with the negative effects of a 10" piece of tubing hanging past the true muzzle, but it
would be legal on a lower receiver with a butt stock.