As
@hawkeye2 said, get a 1/4" or 6mm 1/4" drive hex socket of a quality brand such as Lowe's Kobolt or Craftsman.
You need a workshop with a sturdy vise, files, 2 dowel rods about the diameter of the cylinder mouth and a good hex drive handle.
I have made a nipple wrench from a 6mm socket with 1/4" drive as the 6mm is slightly smaller in external diameter than the 1/4". For a pistol you need the smaller size to fully cover the nipple. This is one of the few times that I retrieved my Dremel Tool from the safe deposit box to work on something.
Measure the distance between the flats of the nipple. Carefully scribe lines on the face of the hex drive socket wrench to give you the most of the internal flats to engage the flats of the nipple. Measure the maximum diameter of the nipple. Measure the length of the cone of the nipple. Using the disc shaped cutting wheel of your Dremel Tool or a hacksaw to carefully cut grooves just inside the scribed lines to the depth of the cone length. Keep cutting away the material until you have a slot that is beginning to look like a nipple wrench slot. Use needle files or a narrow file to remove the remaining metal from the groove. You may need to drill a hole in the center of the socket to accommodate the cone. Fit the wrench to a nipple to verify that you have full engagement. Once you have full engagement, you can prepare to remove the nipples.
Use the dowels in the cylinder to support the cylinder in the vise. One dowel in opposite cylinders. Put the tool over the nipple and with a small ball pein hammer tap and turn the nipple wrench. The impact should break loose the remaining corrosion and the nipples should back out.
I had found a long time ago this little impact tool at Harbor Freight to use. Its no longer listed, but similar tools can be found on Ebay.
You can see my modified 6mm drive socket at the top of the picture.
My tool worked very well in removing extra tight nipples in a revolver I purchased.
If you have the cylinder supported in a vise, you may be able to use your light weight tool and tap and turn to get the nipples to back out.