Kroil in a can, jar, bucket, something.
Stand barrels up, muzzle in liquid, soak for a few days, heat and try to move. If no movement heat and dunk them back in the Kroil for a few more days.
Go about 5 or 6 threads below this one and see what was done to get the tube out of this gun.
This is good advice. As others here said, be very judicious with the heat, if it comes to that. I have freed lots of chokes like this.
OTOH, if you find the constrictions are generally producing patterns you like, leave it alone, or at least avoid the heat.