beag nut, I freely admit that I am new to shooting C&B, so this IS just from my own (limited) experience. The very first pistol I got was a very used Armi San Marco (I think; there is no manufacturer named on the pistol, only "SM" with the proper Italian proof marks) 1858 Remington from a "pawn shop" (it was abused - it had been fired, but wasn't even cleaned before it was sold to the store owner); it wasn't really a pawn shop, more of a third-hand "thrift" store. I cleaned it up, took the cylinder (with nipples installed) to a "professional" to get some caps; this "professional" even told me to use #11 caps. I went out to an out-of-the-way spot I know and blew out the empty chambers with #11 Winchester "Magnum" caps. The first cap had zero problems -- because it was the first to be fired. By the time I got to the fourth cap, they were starting to fall off. Mind you, this was with EMPTY chambers!
I re-seated the remaining caps, making double-sure they really were seated this time. After firing off the remaining caps, I went home, did some reading on the subject of loose caps and took a close look at the nipples: They were ALL deformed, with just the barest hint of mushrooming, likely caused by that previously mentioned abuse. I didn't have to measure the nipples. You may be an engineer -- but I have worked as a machinist in the past, and I know what to look for in a taper. Those #11s didn't fit correctly because they were being expanded by the mushrooming on the end of each nipple. I admit that I didn't know any better before, but I don't know how that "professional" I had talked to missed those deformed nipples....
I have since purchased a new Pietta 1858 Remington, supposedly with nipples designed for use with "#11 caps". Turns out that #10 Remington caps work best with it -- and no #11 cap I was able to try would fit correctly. Shot three cylinders, each chamber loaded with 30 gr. powder and .454 ball, in that pistol, and ZERO caps have fallen out and ZERO caps have been shaken loose by recoil. Oh, and this is with the nipples the cylinders came with.
My point, beag nut, is that although you are an engineer, and there might just be a "better" way of getting caps to stay on the nipples, the most efficient way seems to be to just find the cap/nipple combination that works best for you. By the way, I have already had to remove caps without firing a chamber, so I could put the pistol down "unloaded" (similar to removing the ammo from any other firearm); having some sort of "locking system" to keep caps on nipples, such as you have already described, would make doing that far too difficult and time-consuming for the "average" shooter.
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Engineers may be the ones who design things, but it is the mechanics and users of those things who must make them work...........