Moon Clips
Full disclosure, I don't have a Redhawk, or a S&W model 25. What I do have is a Ruger Blackhawk .45Colt/.45acp, a Webley Mk VI converted to use ACP brass and experience with the S&W and Colt 1917 .45acp revolvers.
My Blackhawk has separate cylinders for .45 Colt and .45ACP. Clips cannot be used, due to the design of the gun.
My Webley uses clips. (S&W) Half Moon clips work, and the 2nrd clips I found at a gun show (no idea who made them) work. I have not yet found a full moon clip that works in my gun. The ones I tried were all too thick for MY gun, which was converted to use .45acp brass, and just barely. .45 Auto Rim brass will not work in my Webley, the rims are too thick!
Most revolvers made to shoot .45ACP have chambers like the semi auto pistol does, meaning that there is a ledge at the front of the chamber that stops the cartridge from going forward. The case headspaces on this ledge.
When this ledge is not present, the case has to headspace off something else. The clip serves that function in guns that don't have the ledge in their chambers.
When you have a .45ACP gun made with the chamber ledge (or shoulder if you prefer), you do not
need clips to use the gun, but, you will
want them with a DA revolver. In a regular DA revolver, without clips, the ACP round will headspace on the chamber ledge, and fire properly. But, it will not extract, because the there is nothing for the DA cylinder's extractor to push on. Fired cases must be poked out one at a time, which is exactly what happens shooting and SA revolver (where clips cannot be used). The SA gun has the ejector rod built on, the DA gun requires you to come up with a tool separate from the gun.
BUT, when you use clips, the DA gun's extractor pushes against the clip, allowing simultaneous extraction and ejection of the ACP cases as if they had rims. This is the primary function of the clip in most DA ACP revolvers.
MOST, not all.
If you have a gun made without the chamber ledge at the front of the ACP chamber, then you NEED clips to headspace the ammo for firing, as well as providing for simultaneous extraction.
Now, as I said, I don't have one of the dual caliber Redhawks, so this is my best guess, but its an educated guess, based on my experience with .45 Colt and .45ACP revolvers.
The Redhawk cannot have the ledge in the chamber to headspace .45ACP ammo. If it did, then .45 Colt ammo would not chamber. SO the gun must use clips to fire .45acp ammo.
By the case drawing specs, the .45 Colt case body is 0.007" wider (its full length) than the .45ACP case is at tis widest point. (.480" vs. .473")
So, what happens shooting ACP in the Colt spec chamber is that the clip supports the ACP case, holding it in the proper position to be fired, which is otherwise literally "hanging in space" in the larger .45 Colt chamber.
Others say the Redhawk only takes Ruger clips. They are probably right. What clips will work depends on how much space is cut in the gun for them to fit into. S&W's use the space designed in the 1917 revolvers, and this also allows the use of Auto Rim brass in those gun.
Ruger is under no compulsion to use that same dimension, and likely does not. If they designed their gun with the space even a few thousandths "thinner" then standard (S&W) clips would not work, and I think its likely that's what they did.
..the long jump in the chamber saps velocity from the .45 ACP in the Redhawk.
I'm curious, what do you base this statement on?? I have never heard it before, in reference to the Redhawk, or any other revolver shooting .45ACP.
I have heard people say that the long bullet jump from the shorter case reduces accuracy (not velocity), and that may be true in some guns, but I have not found it to be the case with mine.
I do think it likely that ACP ammo will be less accurate than .45 Colt fired from a dual caliber Redhawk, but not because of the longer bullet jump, but because the acp case is not supported by the larger Colt chamber, only by the clip at the back end.
For a system that lets you shoot ACP ammo from your .45 Colt Redhawk, it does work. As to the optimal system for shooting .45ACP from a revolver, I don't think it is.