The .45 Auto Rim cartridge cannot be used in either the .45 Colt cylinder nor the .45 ACP cylinder, in unmodified Ruger Blackhawk cylinders. The .45 ACP cylinder does not allow sufficient headspace for the think rim. This because the cylinders did not require the half moon, or full moon, or quarter moon, clips, so no headspace allowance was provided for that. Some cylinders have been modified (aftermarket) to permit seating of AR cartridges. Since .45 AR has now become somewhat scarce, the practice is of little use today.
Bob Wright