VERY GOOD QUESTION!
Here are the two reasons the South did not opt for "High Firepower" weapons:
LOGISTICAL- First of all, many military men at the beginning of the CiviL War were convinced that the troops in battle (and armed with muzzleloaders) fired "too fast" already! So, there was genuine concern that the troops would shoot up all of their ammo before the battle was "half fought!"
Supplying your troops with adequate ammo is a "Logistical" problem.
Of course, the modern American military has conquered the "Logisitcs Problem" [i.e., delivering the necessary supplies to the troops at the Front in adequate quantities] by the use of ships, airplanes, trains and trucks. To the Civil War commander, his army WAS SUPPLIED BY SLOW MOVING WAGON TRAINS away from the rail head or river port.
So, you can see that there was quite a difference between what can be delivered by wagon train VS modern, 21st Century, methods.
TECHNOLOGICAL - From 1863 ownward Southern military units were capturing Spencer Carbines BUT THE SOUTH NEVER DEVELOPED THE ABILITY TO "DRAW" RIMFIRE SPENCER CARTRIDGE CASES! So, many of those Spencers were simply turned in to storage because Southeen made ammo was not available for them.
Matter of fact, when Lt. Colonel James Burton, Superindent of Confederate Armories made his famous "buying trip" to England 1863-the "item" at the top nof his list was to purchase was Spencer cartridge case making machinery. Guess What? The Brits didn't have the foggiest idea as to how Spencer cartridge cases were made!!!
The following year when the .577 Snider was officially adopted by the Brirish Army-Snider "cartridge cases" were made from a IRON base WITH A WRAPPED BRASS STRIP making up the sides of the "cartridge case." Such a cartridge case in a Spencer would not have worked for several reasons.
Matter of fact, the "modern centerfire cartridge case" did not even "evolve" until decades after the Civil War was over!
Immediately after the Civil War the "Benet Inside Primed" cartridge case evolved and only later was the modern "centerfire" cartridge case invented.
Wartime and immediate post wae cartridge cases WERE MADE OUT OF COPPER. Copper cartridge cases have a tendency to "stick" in the chambers of hot weapons-ask General Custer's troopers about that one!
Modern "Brass" cartridge cases did not even "evolve" UNTIL MORE THAN A DECADE AFTER THE CIVIL WAR. Unlike copper cases, hot brass cartridge cases are easily extracted [within limits] rom "hot" guns.
Last but not least, evev IF the Confederate Ordnance Department had had the "technical expertise" to manufacture modern, drawn, brass cartridge cases-after Mid-1863 when the Copper Hill region of Tennessee was captured, the Confederacy DID NOT have had the copper supplies necessary to manufacture cartridge cases.
By 1864 the Confederacy was so desperate for copper from which to make percussion caps they were sending out agents into the hills of Northern Georgia, East Tennessee, etc., to beg, buy or seize copper moonshine stills!
To put it another way-while the Confederacy, provided with blueprints, could have manufactured M-1 Carbines for general issue; they did not have the technical ability to manufacture the ammo! [to "draw" brass cartridge cases!]