Quote History Quoted:
The gap at the aft end of cylinder would be increased for moon clips. The forward gap ought to be as small as tolerable for function.
If the cylinder was machined, then setting the barrel back is no big task.
If moon clips aren't used, the cartridges are unsupported under the firing pin blow.
View Quote
That's not how it works. For a rimmed cartridge, like the .38 Special, .357 Magnum, .41 Magnum, etc., when the cylinder is cut for a moon clip a "floor" is milled in the cylinder and in the ejector star to make room for the moon clip. However, the outer portion of the cylinder is left uncut so that the cartridge can still head space on the rim if the shooter elects not to use a moon clip. In other words, the rim is still fully supported around about 1/3rd of cartridge's circumference.
There will be a little room between the ejector star and the rim (equal to the thickness of the moon clip, plus some minimal clearance), but the ejector star still has the same effect of ejecting the cartridge cases and the shooter will never notice the difference.
For a rimless round, like the 9mm Luger, .45 ACP, etc, a moon clip must be used for ejection purposes as there is no rim for an ejector star to grab. However, if you were to fire those rounds in a revolver chambered for that round, it would still head space on the mouth of the case and there'd be no issue with the hammer firmly hitting the primer.
The exception is when you take a .45 Colt or .45 S&W Schofield revolver and have it cut for moon clips for .45 ACP. In this case, the chamber mouth is a long ways in front of the .45 ACP case mouth, so the only support it has for head spacing is the moon clip itself.