There's a button on the front of the front-sight that keeps the muzzle brake from turning, dont forget to push it in when you're loosening the muzzle attachment. I didnt realize it was there and broke the barrel nut in half (along with a weakened spot weld) when I tried to turn it with pliars when I went to replace mine with a slant muzzle brake. Beginner's mistake
Thankfully no harm done to the rifle.
There is a newer muzzle brake from Primary Weapon Systems.com its called the FSC47. Supposedly its better than the J-Tac-47, but I dont personally know for myself.