If no luck after the boiling water treatment or doused in penetrating oil overnight, try this.
Put the barrel in your food-freezer for a day...... then crank on it. Boiling water expands the metal and that sometimes cracks the seizing seal. Barrel temps around zero degrees do the opposite.... contract the metal and that may give you a fraction of relief.
I've always had the best luck soaking the barrel in Kroil overnight, then putting the barrel in a vice and using a socket and ratchet, constantly tap the end of the ratchet with a hammer, never more than moderate contact.