Normal SCAR 17 flash hiders are pretty easy to take off with the jam nut design but the 13” is another animal.  There isn’t enough space for my normal barrel clamp which meant drilling out chunk of oak block to make a special length.  Managed to break my first two blocks cranking down hard with the vise as the muzzle break seemed to be damn nearly welded in place and it wanted to slip.   Tried the usual tricks heating it up in case of Loctite and then boiling it in case of Rocksett along with putting stuff in to give it better grip.  I just couldn’t get enough clamping on the barrel to keep it from turning.  The problem with the longer section of the barrel behind the gas block it that it’s tapered and it’s a lot more work to make a block suitably tapered to clamp on there.  

I was almost ready to cut the flash hider off but the last trick finally worked... slathered up the inside of the barrel block with the quick setting JB Weld and clamped it up in the vise and let it sit for 20 minutes or so.  Put the wrench and breaker bar on and with a solid crack the flash hider finally broke free.  The JB weld doesn’t actually stick that well to the barrel so it was easy to knock the oak block sections off and it didn’t leave any residue.  It just give enough sticking power to keep the barrel from slipping.

I just share this if anyone else is having trouble... threads underneath were clean and dry with no sign of adhesive, it was just on really tight.

Yeah. Factory FN flash hiders square off on the muzzle end they just craaaaaank that sucker on...

My smith said they probably use 80-90ft lbs...