Here's a thought - cut off the "end choke". That is the barrel at the muzzle that is cylinderical in shape and usually the same length as a shot column. If you cut off the end choke, you will be left only with the tapered section of the choke which is more easily removed, either with a reamer (expensive) or with a brake cylinder hone spun in an electric drill. The hone I used was the one with flat stones, not the little abrasive balls. I have used the brake cylinder hone system on 3 shotguns, which worked perfectly when 'adjusting' chokes and load developmet. It will take some time to get all the choke out, but it's possible. When the stones wear out, merely glue on 60 to 80 grit emery cloth.