I'm an old guy so I'm probably wrong.

I base my opinion with regards to the position of the cylinder during the millisecond period immediately following ignition on physics.

My opinion about the the growth of what we all call "end shake" has been formed by observing and repairing the damage that inevitably occurs to the end of the barrel of the yoke while working as an armorer over the last 40+ years. This peening (shortening) of the yoke happens during heavy recoil....there's really no other way the damage can occur. This gage, or space between components (the end of the yoke and the bearing surface inside the cylinder cavity) will grow from use. That's a very hostile environment in there, and the damage creates not only stress for revolver owners, but a market for toolmakers and the folks who sell yoke shims.

And, you are correct, the shell casing is pressed all the way back in the cylinder under considerable pressure at some point during ignition, otherwise, we would not see breach markings (tool marks) created by the high pressures on the primers and casings.

Perhaps you are also correct about the wider B/C gap causing a loss in velocity, I just don't know. With the standard deviation and variables in hand-loaded and factory ammunition, I would think it would be nearly impossible to verify or quantify such a small, negligible loss of velocity using a chronograph.

I do admire your curiosity and your desire to ask questions. That's how we all learn about these mechanisms.

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Ret. LE, FA Instr, S&W Armorer