Even if its metal, you should still be gentle.
I suspended my flashhider in hot water for a good 15 mins (3 refills of hot boiling water), and flamed it with a cigarette torch lighter . Still would not budge, so I took a clamp and turned it.. resulting in getting a scratched/deformed flash hider.
Good thing the supressor I bought includes a flash hider
Edit: And a note, when you take the flash hider off, take off the excess glue off the outer barrel while you can.