A very slight taper allows the cap to start easily onto the nipple and become tight as it is fully seated. I don't know what kind of nipple you are speaking of, some older designs did indeed have a great deal of taper. Caps of different brands do vary in diameter but I have no problem with the fit of CCI #11 caps on Ampco, spitfire, or Hot Shot nipples, they go on easily and stay put.
Just some thoughts indeed! We use the little plastic cap guards on all of our nipples. CVA had them for a while and they may still have them. We called CVA and the gave us the name of the outfit that makes them. Pictures of them can be seen in the ROA sticky....I think. Or in tech notes at Dixie. These cap guards help keep damp out and keep the blast in. They fit down over the cap and onto the nipples.....James
Think they want the cap to fit tight on the taper, it kind of acts as a limiting device. If it were straight, you'd be mashing on the priming compound pretty hard. Doubt you'd set off modern caps with your thumb, but period caps were not always the best, used different compounds.
the other reading of your question makes me think of this answer: so the baby will let go easier.
]What I was thinking of was a nipple with a straight neck, and a slight chamfer to the edge.
With the tapered neck only the outside rim of the cap makes contact with the taper of the neck. I'll try to attach a crude drawing to illustrate.
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