what ever you do. do not shoot the gun without a choke installed. I had to do that.
It sounds kinda like you already shot it without the choke...
I hope not but if you did try to install the choke SLOWLY backing it off if it becomes tight before it bottoms out. use oil, if the choke does not bottom out do not shoot it with the choke "as far as it will go" this will make the problem worse. You may end up blowing the choke out the end causing damage to who knows what and wrecking the barrel.
You may have to send the barrel in to clean up the threads. Briley does a good job.
I found a spare fixed modified barrel for my BPS 20 ga. on ebay for like 60 bucks.