A lot would depend on what sort of general use. I mean, do we mean humans in general, or big game hunters in general? The latter is, I suspect, what we've been talking about--and the .264 has always primarily been a big game cartridge. If we're talking big game cartridges, the .30-06 is probably the most generally used on the planet. (And among recent centerfire cartridges, the WSSM's are probably the most obsolete....)
For a couple of years I've been thinking about an article on centerfire rifle cartridges that were chambered ONLY in one factory rifle--and if that rifle wasn't produced anymore, then that round would have to be considered obsolete. The inspiration for the idea was the .348 Winchester--but even it wasn't chambered only in the Model 71 Winchester (and Browning reproduction) as apparently some European double rifles were chambered in .348! Which makes sense, as it's a rimmed medium-bore that would be perfect for a lot of European driven game.
Odd you should bring up the .43 Egyptian. I almost caved the other day when a Rolling Block .43 Spanish showed up on the used rack at a local store, in good shape with a very shiny bore. But which .43 Spanish cartridge is it chambered in, and which version is the most obsolete? I'm not put off by having to jump through some hoops to handload some older cartridge, but after some research this one looked like more of PITA than it was worth.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
