On a Smith & Wesson .45ACP revolver (M1917, Model 1950 or Model 1955 Target) what is the dimension with tolerance of the gap between the recoil shield/breech face and the rear of the cylinder? And what is your source?

You can get at the headspace for a .45ACP or .45AR revolver in a roundabout way. Kuhnhausen says that the Go/NoGo specs for un-recessed revolvers are 0.060"/0.064". That is for ordinary revolver cartridges. Rim thickness on .357, .41, and .44 cartridges is 0.060". Rim thickness on the .45 Autorim is 0.090". That would yield a Go/NoGo spec of 0.090"/0/094" for the .45ACP/.45AR revolvers.

The spec allows up to 0.004" clearance between the cartridge rim and the recoil shield. I have found that 0.008"-0.010" still gives reliable ignition without danger of deforming the cartridge base. A little more clearance is a good thing on a defensive pistol - you don't want powder residue, rim nicks, or slightly high primers binding up a weapon that your life depends on. For the same reason, I prefer a slightly larger B/C gap of 0.006"-0.008". Endshake should be no more than 0.001"-0.002".

If you're talking target/hunting guns, the tighter specs are fine.

Buck

The NRA book on Handloading(out of print) has the SAMMI spec. dwg. of the 45 Auto Rim case and chamber. The headspace dimension is shown as .090" min. - .104" Max.

The cartridge rim thickness spec. is .089-.010".
This means the rim can be anywhere between .089" and .079"
So the theoretical head space could vary between .001" and .025" and with everthing still in tolerance

WOB