You normally need a conversion cylider for a 45ACP/45Colt revolver mainly because of the rear face of the cyliders will have to be different. The rim on a 45Colt is much thinner than the distance of the moonclip+the "rim" of the .45ACP. The rear of the .45ACP cylider has to be machined forward of where it would be for a 45Colt. A simple comparison of the rim thinckness of a 45 Auto Rim (which is designed to be used in a .45ACP cylinder) and a 45Colt will show you why. Basically you need twice as much of a gap for a .45ACP than you get in a 45Colt revlover.
The 45Colt IS a great round, however it is just this side of too big. It was desingned for black powder originally and with modern powders, there is usually excess space left. Generally speaking any revovler that you're going push hot 45Colt loads through, you can push hot 45ACP loads through that have similar performance. You could probably shoot longer and heavier bullets out of the 45Colt, and it would probably be a better hunting round as well. Still, if you need a high-horsepower revolver round, a 44 or 41 is probably a better choice than hot-loading a 45Colt.
The 45ACP round is a superior revolver round because it's loading system is faster (i.e. the moon clips load 6 rounds all at once, not as fast as a mag change mabye, but faster than single rounds). The shorter case also allows easier and faster ejection. The 45Colt rim is not a large diameter one, and it will sometimes get overridden by the extractor star and leave a case in the cylinder. With shorter barrelled guns, this becomes important if they have a short throw extractor. With the clips, there exctractor override is unlikely.
There was a S&W factory .45ACP/45Colt M25 (I think the M25-3). It had two cylinders. I've seen conversions that worked the same way, but there is a difference in the 45Colt cylinder and the .45ACP cylinder on the M25. The .45ACP cylinder is set back from the front of the frame, the same distance as the M27/28 .357mag cylinder is. The 45Colt cylinder is full length, same as the M57/29 cylinders in .41/.44 are. You can't just swap a M25 cylinder of one .45 caliber into the other. The cylider has to be custom made, either a .45ACP cylinder made for the 45Colt gun (out of a .41 cylinder) or a .45Colt made for the .45ACP gun (out of a .357 cylinder)
The Ruger Single action is a very simple system as it allows easy swapping of calibers and the single action design with the loading gate and ejector rod negates the need for moon clips.
If you're all set up for .45auto, then I'd go with a S&W M25 in .45ACP. You can really have alot of fun reloading the .45ACP in an auto because you don't have to worry about it feeding or functioning. The revolver does that regardless. You can shoot reloads that would never, ever work in a revolver. Also if you are thinking about snake shot, use cut-down .308 cases clipped into the moon clips. You can stuff alot of shot into one of those. You may not get a .410 equivelent out of it, but I've seen it done with '06 cases (and the part of a .308 case you'll use is the same) and it's far better than the store bought snake shot. Once you shoot it, the case fireforms to the cylinder shape, headspace shoulder and all, and you may not even need to use clips after that. Think outside the box when laoding for a .45ACP revovler (just do it safely).
Get the .45ACP M25. You won't regret it.
Ross