I just tried my new Ruger stainless Flat Top .45 Colt/.45 ACP convertible out with the ACP cylinder, and I had problems with the cylinder binding up due to brass backing out of the chambers as I loaded it. This was Federal American Eagle 230 gr FMJ ammo. Maybe it is an issue with the muzzle not being pointed down while loading, but I have never had this issue with any other Blackhawks I've owned and still own. If I made sure the cartridges were fully seated, kept the muzzle lowered and shut the loading gate before raising the muzzle...there seemed to be no issue. Anyone else run into this?

Unfortunately, I did not have the .45 Colt cylinder or ammo with me on this trip, so I will check that out next weekend.

Might need to have your cylinder reemed out. Ruger is notorious for under sizing the cylinders on their .45 caliber Blackhawks. My local Gunsmith charges $25 bucks to do it, and it also helps with the accuracy. I've had it done on all of my .45 caliber Blackhawks.
Also, you can buy the reamer from Brownells & do it your self for about $45, I think.

I would take a set of dial calipers & measure the Cylinders, first, though. For more info, I'm pretty sure that Hamilton Bowen mentions the above procedure on his websight.

One last thought. It could have to do with the 45acp case being "rimless" as compared to a 45 LC case, which seats in the cylinder better due to being a rimmed case. If its not binding while shooting with the .45acp cylinder, it could be just as simple as pointing the muzzle straight down while loading. It should not be an ammo issue with the Factory Federal Ammo. I shoot that cheap stuff all the time through my .45acp / .45lc Ruger Flattop with no issues at all. Hope this was helpfull.

I have the same trouble with the same gun! I now just shoot my other Blackhawk .45 (1977 vintage) with its ACP cyl. I am interested in what the fix is also.

I guess I will contact Ruger and see what they have to say. I was curious to see if anyone had the same issue.

You really just need to hold the gun muzzle down when loading. I can bind my 45 colts if I load it with the muzzle some what up.

This is less than "somewhat up" - this is anything more than about 20 degrees from vertical pointing down. I have had or still have a half dozen Ruger Blackhawkks or Super Blackhawks in various rimmed calibers and never had an issue with loading the gun fairly horizontal in my left hand. I'll see how it works with the .45 Colt cylinder before talking to Ruger.

Let gravity work for you, not against you

Bingo. All of my older Blackhawk .45 ACP cyls have no trouble like this NM Flattop. Something is amiss in the cylinder for sure.

I bought a Bisley 45 convertible a year ago and never had an issue shooting ACP out of it. I've used the ACP cylinder a lot in teaching my wife and daughter to shoot.