Small metal can,with 3 inches or so of kroil in it,hang the gun with the barrell down in the can,and leave it for a week,For heat I would shoot a box of cheap lead 8 shot shells through it fast,and then get after the choke with the choke wrench,twist until it stops and then screw it back in,basically what your going to do is recut the threads with the old threads,If you manage to get it out make sure to grease the thing on regular occasion from here on out,the next time you may find yourself with a fixed choked barrell forever.My buddy made this very same mistake with a ruger red label over/under the gunsmith who refinished it couldn't get his out and now it's fixed imp cyl and modified.As a welder I can say there is another option with cold,you could head to the welding store and get a thermos of liquid nitrogen and freeze the choke from inside the barrell with a qtip on a stick,there are a lot of things we get unstuck at work that are similar,we use either liqud nitrogen or Ice,you could preform a cube of ice with a tube of same inside diameter as your choke and send it to the muzzle from the breech,the trick is to wait just the right length of time,if you wait to long the outside will constrict as well and put you back at square one.