Greetings,
The B21 is a very nice rifle and one that wants to be worked on. I bought one probably 20 years ago and at that time I wasn't so busy and really enjoyed working on it. Thought it didn't have the breach problem yours does, I was tempted to re-barrel it in .20 caliber just for fun. I didn't follow through with that project, but I did come up with what I thought would be a way to manage it. Before I describe this idea, I'd suggest you look into working on the breach of this barrel from the back end, not the muzzle end. Working from the muzzle will ruin the rifling. Removing the trigger assembly, spring and guide, piston and compression tube is pretty straightforward with this rifle. Then you can access the breach of the barrel from the back end. Using a cleaning rod you can hand turn a tool against the breach of the rifle and remove as much material as needed.
It won't be easy to do a great job with this project, but the accuracy of your result will tell you if you have succeeded. My idea for re-barreling would be major surgery, but wouldn't require as much precision as what you are trying to do by hand. My idea is to leave the large original barrel in place and drill it out to accept an insert of a new barrel.
I didn't complete this project because my B21 responded very well to a shortening and crowning of the barrel along with a few other mods, so i can't tell you everything you would encounter in this project. If opening up the breach of your existing barrel works then that's great and you will have a powerful .177 cal rifle and the pride of having made it work well yourself.
Good Luck