Before you do any cranking on the flash hider, you MUST use a barrel vise or else you can wreck the upper. Also, once you get barrel secured, use the correct sized wrench, not a crescent wrench or pliers.
Give the wrench a gentle rap with a small hammer going to the left, facing the muzzle end. It will come loose.
When you replace it later use a crush washer and tighten the f/h snug. Ta Da.
Hobie