Soaking in 50% Auto Transmission fluid and 50% acetone is about the best penetrating oil you can get.
Don't get in a hurry let it soak,.

When soaking, I would set it up so the barrel is vertical and then also pour a few ounces down the barrel to let it soak in from that side of the nipple also.

If God wanted you to walk and carry things on your back, He would not have invented stirrups and pack saddles