Neat ideas from alternativists. A few points to consider:
* The American Civil War/War of Secession/War between the States was fought primarily with muzzle-loading black powder weapons, swords, and bayonets. We always start wars with the last war's technology.
* As pointed out in other posts, the South/Confederate States had a shortage of brass already, adding cartridge cases to that shortage would have buried their industry.
* The detachable magazine was not invented until the turn of the Century.
* Black powder does not lend itself terribly well to self-loading weapons, let alone blowback weapons. A weapon that could load and fire 5-10 shots before jamming would have been worse than useless.
* It took a genius like John M Browning to observe the effects of fugitive gases and apply a mechanism to capture and use them to cycle a firearm action. The first practical use of self-loading was in the 1880s, although the Gatling was available at the time of the War.
* Like any other technology, changes come fast and furious during wartime, and the Civil War saw the progression from pecussion muzzleloaders to cartridge repeaters in a very short period of time.
* There was so little time to develop and apply the new/developing technology, and we see samples of lever action rifles appearing very early in the war, but acceptance, manufacturing capacity, and cost considerations kept them from becoming major players.