50% ATF and 50% acetone.
Dunk the action in the stuff and let it sit for a few minutes.
Then attack it with a hot air gun. The "home brew" is an old machinist blend for stuck bolts and it's been tested over/over and proven to beat Kroil, WD, etc.
The hot air gun will loosen things up without fear of ruining heat treatment or surface finish.
For barrels that are attached with a bit of grunt it takes purpose built tools for the task. if you don't have them, either buy them or take the job to someone who does and knows how to use them properly. It's not worth ruining an action or a barrel.
Good luck!
Chad
PS:
Regarding galled barrels/actions. It IS POSSIBLE to remove the barrel without destroying things or having to result to cutting the tennon off and machining it out of the action. It's not always possible, but I know it can be done because I've done it.
It's complicated to explain and takes a great deal of patience and even more luck. I've done a half dozen this way on Nesika actions barreled by other smiths.
Basically you warm it up and slowly work it back and forth. With time and luck it'll wiggle past the sticky part and you'll get it off Use heavy dosages of Kroil, the ATF trick, or even some grease if you can fiddle faddle a way to get it in there. (a zerk adapted to a base screw might work. . )
Good luck.
C