Are you talking about gas rings? The extension on a cylinder that covers the cylinder pin, as the bushing does on a Colt SAA? The 1872 Open-top0 has one, the first time Colt used the idea. It keeps the cylinder gap blast out of the cylinder pin by giving it an offset. To install, the cylinder would have to be counterbored 1/2 inch diameter for a short distance, probably about 3/8 inch, and the tube pressed in, 1/2 inch O.D. and the same I.D. as the cylinder base pin bore. The the barrel would need a cut, also to clear the 1/2 inch diameter tube, at the bottom of the barrel throat extension concentric to the cylinder pin hole, up to the face of the barrel lug. The length of the gas ring/tube needs to be the depth of the cylinder counter-bore plus the (desired cylinder gap shorter than the) distance from the cylinder face to the lug face. If that isn't clear enough, ask and I'll try again. I intend to do it to my Remingtons as well as my converted Colt Navies to extend the shooting time before fouling stops it.
