WD40 is the worst you can use.
Go purchase a good grade of "wrench ease" and soak the barrel tip as described above for a dau or two.
If that don't help go see a gunsmith.
Now to prevent that in the future. I use a thread lubricant specifically made for high tempteratures. It is a small squeeze tube (can't remember the name) - I apply a small dab each time I clean the gun.
More than likely you got a bit of moisture there - not a probelm I had a few stuck to until I started using the choke tube lube.
Oh and I also went to Briley choke tubes for most of my shotguns and they are extended. Extended allows one to put a pliers on them if needed.
And I also bought a better choke tube wrench tht fits all 4 notches and not just 2.
JW