Having read all the other posts, you can also use BP Blaster and soak the muzzle end for a few days and then try the choke tube wrench and see how it goes while having someone hold te barrel while you turn it.
If none of that works, try this. Place barrel in freezer, seperate from rest of weapon please, for 2 days. Take out and IMMEDIATELY place hair dryer on "High" and rotate barrel to cover all the way around choke tube area of barrel for several seconds, 30-45. and then take choke tube wrench and try removing while someone holds barrel for you. Theory behind this is that the frozen choke will stay smaller and seperate away from hot barrel and allow removal. I have done this for several buddies and it seems to work. If all fails, take to reputable gunsmith. Then buy extended choke tubes and problems go away.