2 things to do that might help if shooting the barrel hot dosn't work

This may hurt
But I would drill a hole through the exposed part of the choke tube , a good drill press and a colbolt(sp)drill bit
So a rod can he inserted through and used to turn the tube out

I like to soak the end of the barrel/choke in Kroil or hot ATF ( heated on a hot plate to about 120 or so degrees with the barrel hanging in it )
I soak the choke tube for a day or 2 and try turning the tube out
If no luck , let it soak for a few more days

I made up a pair of vice blocks out of hard wood , lined with leather , with rosin added
To hold the barrel
As if you try and just hold the gun , no real way to do it safely
And never try and use the barrel ring to stop the barrel from turning in a vice , as I have seen several barrel rings bent or broken off

John