I took one out of a friends blackpowder rifle. He had cleaned it since he shot it a year earlier.
That thing was crusted with rust. I used PB blaster to get it loose. One spray, set for 10 mintues, came out no problem. I had tried to remove it earlier that day and bent the nipple wrench without it budging. That was the first time I had used PB Blaster. Now I use it all the time. It's awesome. You can get it at Lowe's.