Something to keep in mind about clamping a cylinder in a vise, even with wooden jaws is it's real easy to crush black powder cylinders. Especially if the jaws are located over the chambers.
The cylinders are not made out of modern high strength steel like a modern revolvers cylinder is.
I mentioned a safer way to keep the cylinder from turning. For those who don't want to try to find it I said,
"By the way, to keep the cylinder from turning while you are trying to unscrew the nipple, if you have a vise, do not clamp the cylinder in the vise. Find two pieces of wooden dowel or metal rods that fit the chambers and stick them into two of the chambers that are opposite each other. Then, clamp the two pieces of dowel that are sticking out of the chamber mouths."