Question about taking off an uncommon flash-hider

Unless pinned or welded it should just unscrew. Look very carefully for a spot that may have been ground or a circular mark on the side of the flash hider.

If you don't see one then check if there are 2 wrench flats, possibly (probably?) not parallel. If this is the case there is a lock ring to allow indexing the slots on the flash hider to your preferred position, put a close fitting wrench on both and, while holding the ring (closest to the breech) stationary, rotate the other part counter clockwise as seen from the muzzle.

If there is no weld/pin or lock ring it was likely installed with loc-tite. you will need a barrel vise to hold the barrel and probably heat (propane torch is fine) and a close fitting wrench. Clamp the barrel in the vise, apply heat to the hider for a couple of minutes (it won't get hot enough to affect the barrel) then loosen the hider with the wrench.

If none of this works, get thee to a good smith to save head aches and money.

depending on how lawbiding the previous owner was that should be pinned or welded for a sixteen inch, but if he did it himself you never know, most likely threaded.

You can see the flats on the ring farthest from the muzzle for a wrench, as well as on the brake itself. Perhaps this tightens a collet-like area in the threads. You might try some heat and/or penetrating oil on that area, then close fitting wrenches on both. Turn the ring counter-clockwise as viewed from the muzzle. If that loosens then try un-threading the brake in the opposite direction. If neither works and you just want to lose the brake, you could very carefully use a dremel or files to remove the ring and maybe the brake as well. If you don't feel confident with this, get to a smith and pay the man to do it properly.

That looks to me to be A friction fitting type device to get a muzzle brake on there while the Brady Bill was in place and threaded muzzles where illegal.

There maybe a relief cut or knurling under the fitting, but to get a true AR muzzle brake on there I am sure you are going to have to have the threads turned.

To me that looks to be plumbing or AC fittings made into a muzzle brake