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There have been several threads about this very minor but very useful modification. Some will always ask why do it? And the shortest answer is the proverbial: if you have to ask the question you won't understand the answer anyway.
So you needn't try to answer.
The short answer is 'face off' .089". ACP rounds do not protrude from the Ruger cylinder. But, nothing is ever "never" or "always" in this world.
So double check yours with the cylinder out of the gun and a straight edge using factory ammo (different brands) just to be sure, and check every chamber with several different rounds.
If you have protrusion and it's the same or within a few .001"on all 6 chambers don't worry about it if the cylinder has been shot w/o problems. Just use a caliper to measure the cylinder length with the longest rounds chambered and without. The difference in those two measurements is how much to SUBTRACT from the .089" cut.
Just for kicks let's say the protrusion differs from chamber to chamber by more than a few .001"; it should go back to Ruger to have the shoulders equalized under warranty unless whoever is facing off your cylinder can do it.
There’s more: in addition to your question, there has been discussion about using full moon clips in a Ruger. Yes of course you must remove and replace the cylinder to use them, so why you may ask. Well it might be counterintuitive but many, many SA handgunners use SAs for concealed carry and backup carry while hunting.
And everyone knows a SA reload is the slowest of any handgun. But there is no faster reload in a SA than a cylinder swap or speed load by removing cylinder, reloading with a another loaded cylinder with rounds in a full moon clip (or dumping 6 cases in a full moon clip and speed load the original cyl with 6 clipped rounds ala Jerry Miculek) and popping the cylinder back in. For the average person it's about twice as quick or quicker with just a little practice.
You can shoot the 45 Colt cylinder first and your ACP cylinder can be your backup cylinder fully loaded with 6 clipped rounds of ACP. The clip is a real benefit to prevent errant loose rounds from dropping out of the cyl while replacing it in the gun. That scenario is why the ACP cylinder should be the backup load.
So where do you get full moon clips for a Ruger? You can’t, but Smith moon clips are extremely simple to modify whether you have a mid frame Flat top Blackhawk (and New Vaquero) or the full size Blackhawk. But both chamber centerline-to-axis centerline radiuses are smaller than the S&W N frame cylinder. That’s a good thing because the moon clips load from the outside and can be fitted. First with a Dremel tool, slightly round out the center hole until it slips over the ratchet hub of the Ruger cylinder. Lastly deepen the notch for each round; slightly for the large frame Blackhawk and just a little more for the mid-frame cylinder until a loaded clip plops onto the cylinder with no binding of the rounds as they drop into the chambers and your done!
While you’re facing off the ACP cylinder, think about facing off .044” from the 45 Colt cylinder as well (.045” less than the ACP cyl) and shoot ACP clipped rounds in it as well as 45 Colt! Or for $80 Mark at Pinnacle Guns will alter the cylinder for moonclips so you can shoot .45 ACP through it, and keep the .45 Colt capability.
http://www****gerforum.com/phpBB/viewtopic.php?t=147059
Note: by surface grinding .020” off of a standard steel moon clip (with the above fitting) you can actually shoot ACP in an unmodified 45 Colt cylinder.
Now this may not matter at all to you, and you can just enjoy shooting the 45 AR, but the additional versatility is still there.
I know that’s a lot more than you wanted to know but I hope it helps and all the best in your endeavor,