I keep promising to shut up... but someone asked about the 10mm... Here, in Jeff Cooper's own words... why it's not caught on... (jeff was one of the original developers of the 10mm... He has since determined that it wasn't really a good idea)
"When Whit Collins dreamed up the Bren Ten cartridge back in the dark ages, the idea was to obtain equal or superior stopping power to that of the 45 ACP in a weapon of less bulk. The 9mm P cartridge has never been quite up to serious combat potential since its inception back in 1908, but fitting a truly big-bore cartridge into pistols designed for the Parabellum round did not at once become accepted. The Browning P35 service pistol had much to recommend it over much of the 20th century, but it is not possible to stuff a 45 ACP round into that action. Whit Collins went back through the stacks and discovered that one might get fairly good impact effect out of a 10mm (40 caliber) cartridge, and this proved to be a practical idea. Experimentally in California we were able to get a 40 caliber pistol bullet of 200 grains up to about 1000f/s without blowing anything up, and this gave birth to the idea of the "Bren Ten." This was a very promising concept, offering slightly greater power in slightly less bulk. There were, however, problems. The Bren Ten cartridge, loaded up to its full capacity, tended to be very hard on machinery, and it wore out available locking systems pretty quickly. It also kicked pretty hard. One answer to this situation was to load the Bren Ten cartridge down enough to avoid excessive violence. This resulted in the succeeding rounds known as the "40 caliber Smith & Wesson" and its cousins. A downloaded Ten is probably a better fight-stopper than any version of the 9mm, but it should not be mistaken for a full-house Bren Ten. Many people do not understand this and extol the 10mm Smith & Wesson as a satisfactory successor to the 45 ACP. Things do not exactly work out here, and while the Bren Ten as fully loaded is a pretty decisive service round, the "Attenuated Ten" comes on somewhere halfway between the Parabellum and the 45. This is not a disaster, though it does confuse things somewhat. It is unnecessary to bear in mind that the "Attenuated Ten," while a pretty good round, is not a way to achieve something for nothing"
And, writing on the suject just this (2004) April in a short article in Guns and Ammo, Cooper stated;
"We recently had occasion to discuss the history of the Bren Ten with a correspondent who was obviously more of a collector than a shooter. The Bren Ten was a concept of mine, and while I am not ashamed of it, I admit that this concept was not entirely sound. What the Bren Ten pistol achieved over the 1911 was range. The full-house, 10mm cartridge--definitely not the attenuated 10s that are popular now--pushed the effective range of the combat sidearm out beyond that which is usually expected. But extending the manageable range of a combat pistol out beyond the ability of the shooter to utilize it does not accomplish much. The full-house Bren Ten should be able to achieve reliable one-shot stops out to at least 50 meters, but pistol actions do not take place at 50 meters. The combat pistol is best employed at distances hardly more than across the room, and the Bren Ten will not do this any better than the venerable .45 ACP, or so it would seem."
http://www.home.earthlink.net/~gnappi/
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