Dale,
Thanks for that. Upon further contemplation, I must agree with you. Although I've seen it written for both Colt, and S&W (for S&W in the Journal Book 2 reference to Kuhn, 1961), I can't believe that either Colt or S&W would use a charge hole for a non-heeled bullet cartridge. After all, they had been chambering their revolvers for modern cartridges with chamber throats for at least 40 years by 1917.
If Colt purposely positioned the chamber shoulder deeper than the .45 ACP required, I wouldn't think it was for the .455 cartridge. I know S&W British Svc revolvers are only marked .455 because they're actually chambered for the longer .455 MKI at British request, so both MK I & II could be fired. An ACP falls all the way in the chamber.
Photo by Lee Jarrett
Did Colt produce any 455 New Svc revolvers for the British? If they did, I can believe they may have shaved the rear cyl face of unused cyls for use in their early 1917s. They certainly had precedent for that by using left over 1878 DA cyls in their single action army. They didn't waste anything.
Certainly didn't need deeper chambers to ream them on purpose in ACP cyls. Neither for the shorter MKII or even the longer case .455 MkI (.455 Colt and Dominion) same case length as the ACP. I have both, they do not chamber all the way and no deeper than the ACP in a S&W 1917 ACP cyl chambers because of the tapered case, as you said. So they wouldn't need a deeper reamed shoulder.
It certainly wouldn't be for the shorter MkII, which headspaces on it's rim at the cyl face. It would not fire or not reliably enough with it's thin rim.
The MKI will fire for certain, because it's tapered and too fat at the bas to chamber completely as you thought.
Or were they left over cyls chambered for the .45 Frankford Arsenal ACP equivalent cartridge with a rim, submitted in the New Svc for the 1906 Army Trials???? That would make more sense for the deeper tapered shoulders since that case was longer than both the ACP and MkI case.
The government didn't have to share S&Ws 45 ACP shoulder position with Colt, it was not patented, nor patent-able under patent law. It was not only too general of a concept but one that was already in the public domain that all manufacturers were already using for at least 40 years.
But the gov't did propose that S&W share their clip patent rights with Colt at no royalty expense which they did. And before the Gov't took over production. Smith's original clip design from the 1870s was for a full moon clip, but that's another story.
Like you, I would really like to know the true story.