A few cycles of heating with a propane torch followed by a spoonful of penetrating oil will probably loosen it up. I prefer acetone and ATF mixed 50/50 but Kroil might do it too. Might smack the end of the wrench with a mallet a few times while it’s hot to help shock it loose.
Not need to get anything overly hot, you don’t want to change any heat treat or get anything red hot. Hot enough the oil is smoking off but no more than that.
You might soak it from the back end for a day or two, then turn it over and put some down the barrel from the muzzle and let it soak into that side for a while. Get it from both directions that way.
