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There really was no ".45 S&W" as S&W never made a .45 revolver (up to the WWI era) except the Schofield and it was made for the .45 Government, which had to have a small rim diameter in order to fit the Model 1873 (SAA) revolver. When the Model 1909 came out, the Model 1873 was long out of service and the military could not have cared less whether the Model 1909 cartridges fit the old single action or not.
The rim diameter of the .45 Government is .512" (Frankford Arsenal Benet primed), the .45 Colt is .505" (modern); the .45 Model 1909 is .536" (Frankford Arsenal 12 18). Measurements are from specimens indicated. The .45 Government has a slightly larger rim than the .45 Colt, with some specimens up to .523" rim diameter, about as large as it can be and still get six into the Model 1873 cylinder. That was done to help resolve extraction problems with the Schofield.
While the confusion is rife about various rim diameters, there is little discussion as to why the .45 Colt has such a small rim in the first place. The answer lies in Colt's traditional attitude of never wasting anything. When the Army went looking for a new revolver and hinted that .45 would be nice, the "anything" was all the tooling Colt had invested in its percussion revolver frames. To make the cylinder significantly larger than that of the .44 Army would have meant revamping a whole lot of expensive machinery. So Colt made the cyinder a bit bigger (1.621" to 1.652") which was about as much bigger as it could go without a lot of changes to the grip straps, internal parts, etc. But skimping on cylinder diameter meant a .45 cartridge had to have a small rim, no problem at all with the SAA, which has an ejector rod. But when folks wanted to use the .45 Colt in rifles and other revolvers, the small rim was a problem. It is only recently in response to CASS and other sports that rifle makers have come to chamber the .45 Colt, and it still gives trouble.
Jim
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