Great write up Coffenmaker. I understood everything.
I think the other confusion if not the main confusion is that Open Tops are both Percussion and cartridge, ie, conversions and my made for cartridge 1972 open top.
Perhaps, Percussions are not considered Open Tops??? If that is true then my statement above is from my lack of knowledge.
I was not confused in what you wrote by the way. You made it very clear about Percussion guns however, my 1872 OT has a Gas Ring which is currently acting as a bushing and maintaining end shake. It is "thin" and according to Mike it should not act as a bushing to control end shake because "it's so thin". Mike said.... please correct me if I'm wrong Mike... "that the barrel should control the end shake just as you explained for a Percussion gun, with a .003 Barrel to Cylinder gap "Mikes opinion". That will take care of the Headspace. Mike intends to correct it as such and the gas ring will act as a gas ring only and not as a bushing to control the end shake as it does now.
So, what I'm saying: In my case, my "1872 cartridge OT" will be converted or corrected to a Percussion setup when Mike gets done with it, I assume, as it should have been done in the first place by Uberti.
So you know I know: I understand headspace for rimmed cartridge revolvers. It has to be maintained one way or the other. .060" to .074" I think is the SAMMI spec for 375/38 special cartridges, measured from the breech face to cylinder unless there's a recess for the cartridge and in that case, the recess depth is factored in. .014" is the Maximum tolerance. That's quite a lot of wiggle room for end shake actually unless your headspace is edgidging toward the maximum. Headspace is measured with the cylinder held "forward".
I know nothing about Percussion Cap and Ball if there's any kind of similar requirement.
Likewise, I do not know about the Conversion Open Tops, if they are converted so that headspace is controlled by a cylinder bushing versus the face of the forcing cone at the end of the barrel.
What I did learn from you but before you, my discussions with Mike, is the fact of the barrel face being used to control end shake versus a cylinder bushing. What I'm not sure of is why one would be more advantageous over the other however, I can see that using the barrel to maintain end shake, also controls both the gap and headspace all at the same time, assuming a well fitted arbor.
I believe it said in my owner's manual that if the cylinder is binding when putting it back together, to back-off on the wedge a bit. On mine as is, it will only bind on the gas ring leaving a large gap remaining between the cylinder and barrel.
Please forgive me if you're needing more Aleve. This is all so fascinating to me because I'm new to this stuff and love discussing it. I prefer Aspirin!