Right, here's how it works.
When you have the 45ACP cylinder in hand, hold it up to the light and look in from the rear. There's a "lip" machined into the inside edge of the chambers where the front edge of the 45ACP shell will go. That's how it headspaces...exactly the same as a semi-auto chamber
.
Something else going on here. From that "lip" outwards there's a very, very long straight "throat". The bullet is going to travel through about 3/4" or so of tight "smoothbore" area, and it's going to accellerate STRONGLY within that section. When it finally clears that and crosses over to the barrel it'll be going pretty damn fast. Which isn't a problem in a big tough gun like that, but what it means is, you get a hell of a lot of bullet speed for the barrel length.
I've studied this because I'm converting a 357Mag New Vaquero to 9mmPara and the principles are all the same.
S&W has made 2" barrel factory snubbies in 9mm. When people measure the bullet speed out of them, they seem to spit rounds out about as fast as the same round would clear a 4" barrel Glock or the like. It's a very efficient setup. Accuracy is also surprisingly decent considering what's going on; the Ruger Blackhawks in 45ACP all seem to shoot quite well. Fortunately the optimum barrel spec between 45LC and 45ACP is the same, and Ruger barrels tend to be pretty good. You may find the 45ACP cylinder very useful overall
.
Pro tip: use single-column 45ACP semi-auto mags (1911 or whatever) as speedloaders, thumbing rounds into the loading gate from the mag
. It's not exactly "the cowboy way" but it works great.