When I first started loading .45 Colt, I immediately noticed that the volume of the case occupied by powder was extremely small.
From what I've read, this is the reason they came up with TRAIL BOSS powder. Trail Boss is light and "fluffy" has a lot of bulk for its weight. It is made to take up the "air space" in the old blackpowder cases (.45 Colt, .44-40, .38-40 etc.) and deliver full (standard black powder) velocities.
No need for a special short case if you use Trail Boss.
Just talked to Hunter at Starline, their tech guy. Three points:
a) the .45 Special case is the same as the .45 ACP and .45 AR, except for a different head (i.e., rim).
Is it possible that the tech guy at Starline was only talking about the case LENGTH when he said this?? I think he was, because of the (b) and (c) statements.
It would be very odd for a company to make a case, INTENDED to be used in .45 Colt chambers (just shorter length than the .45 Colt case), and make the body dimensions (diameter & length) identical to the .45ACP, "just with a rim".
length, sure, but NOT width. Some people seem to think the .45Colt/.45ACP thing works just like the .357 Mag/.38 Special or .44 Mag/.44 Special does, but they DON'T. With the Mag/Special interchangeability, it works because the cases are the same dimensions in all aspects, EXCEPT for length.
This is not the "case" with .45 Colt / .45ACP & .45Auto Rim.
Again, I point to the specs, the .45ACP/AR case is 0.007" smaller than the .45 Colt at the case mouth. The ACP is 0.004" smaller than the Colt case at the case head, And the ACP case is 0.480" RIM diameter, which equals the .45 Colt case MOUTH diameter.
SO, why would Starline (or anyone else) make a short (ACP length) case to fit a .45 Colt chamber with the undersize, and tapered dimensions of the ACP brass???? Makes no sense. SO, I think the "tech guy" at Starline must have been referring only to the case length, not all of the dimensions when he said it was the same as the ACP, but with a rim.
Generally these can be loaded using .45 Colt dies and a modified (shortened) crimp die, or .45 Auto Rim roll crimp die.
Here's a point to be aware of, if you buy the short Cowboy brass, you will need a special die to crimp it. Standard .45 Colt dies will load it, they have enough adjustment for that, but they can't crimp it, because the seater die body is too long to allow the crimp shoulder to contact the short case mouth.
SO, either cut off enough of the bottom of a .45 Colt seater die to compensate for the short Cowboy case, OR use a .45ACP die, which might not have a roll crimp in it, depending on the age of the die.
The taper crimp found in modern ACP dies might be fine using the light loads the short Cowboy brass is intended for. Only field testing in your gun with your loads and your crimp amount will tell.
A roll crimp WILL work fine, but today I don't think its built into ACP seaters like it once was. I don't know for sure.
I do know that Lyman .45ACP die sets from the 70s (and older) were made to load ACP and AR, and have a roll crimp built into the seater die for the AR. Not sure what other die makers did.