Look at it real close to see if it has wrench flats on it. If it does, look even closer and see if you can see any small round marks like a drill hole that has been filled. If you find one of those then the flash hider has been pinned and welded and it will take more work to get it off (you have to drill out the pin). If it has wrench flats and nothing that tells you it has been pinned, then apply heat ONLY to the flash hider. If Ruger used Loctite it may take some heat to break it down.
Does it have a crush washer where it meets the barrel? If so that would tell you that it should be able to be removed. If no crush washer or any other washer, and if it does not have wrench flats, look at it VERY closely where it meets the barrel. Look for any sign that it is not part of the barrel. Ruger has used barrels that the flash hider cannot be removed. My SR556 barrel was made with the flash hider as part of the barrel and it cannot be removed unless you cut it off and then thread the barrel.