What would have served confederate infantry better shorter range breech loaders or longer range muzzle loaders?
The only source of copper for making cartridges in the Southern US was outside Chattanooga TN in Copper Hill. The only mill that could turn that copper into sheets was in Cleveland TN a few miles up the rail road.
Without the rail link through Chattanooga, the copper mine & the copper mill, the CSA could not have produced cartridges for repeating rifles. All three were captured in 1863 by the Tullahoma/Chattanooga Campaign. There was no possibility of running cartridges for repeaters through the blockade like caps were. Likewise, the CSA did not have the industrial base to replicate the Union complexes that produced millions of cartridges during the war,
In any case, there was no significant difference in the effective range of Spencer rifles or metallic cartridge carbines & muzzle loaders. As was dramatically demonstrated during the Tullahoma Campaign, rain soaked muzzle loaders could not fire at all under conditions that did not affect metal cartridges.
The direct answer is an emphatic no to the premise of the question asked on all counts. The CSA could not have produced metallic cartridges in volume. There was no difference in the effective range of metallic cartridge weapons & muzzle loaders except during bad weather. Under those conditions the muzzle loader’s effective range could became zero.
It is interesting to note that the effective range & lethality of muzzle loading smoothbore musket balls & arrows launched by archers or mechanical means came down in favor of the arrow. Of course, mastering the heavy pull of a war bow was a lifelong discipline. Almost anyone with four front teeth to tear cartridge paper could fire a musket. It can & has been noted many times that inclement weather could make both weapons useless.
The centuries long record of arrow vs muzzle loaded musket ball contains examples of each having a decisive tactical edge over the other. As the Zulus demonstrated, under the right tactical conditions, hand held edged weapons could be used to defeat trained soldiers armed with metallic cartridge rifles.
Surgeons who treated wounds inflicted by arrows & those by smoothbore muskets stated that arrow wounds were much harder to treat successfully. The bone shattering impact of a rifle bullet gave it a decided advantage over both arrow & musket ball.
Bottom line, of course, is that nobody who falls victim to a projectile weapon has much interest in the comparative lethality of projectiles that did not hit them. John Keagan, the much acclaimed British military historian, notes that the most common objects removed by surgeons from wounds during the age of close order black powder smoothbore musket warfare was other soldier’s teeth & bone splinters. The highly infectious nature of such projectiles gave them an edge in eventual lethality over any other.