Staff Writer (Updated: 2/15/2016):
The Springfield musket was a muzzle-loading .58 caliber weapon, muzzle-loading in that the propellant and the round (a .58 caliber "Minie Ball" shot) was entered via the muzzle end of the barrel and rammed down by way of a ramrod. The Minie Ball itself, being of French invention, was a potent man stopper. It worked well to break bones at nearly any range and could be lobbed a great distance with a propensity to bounce after landing - sometimes inflicting more damage. The Model 1861 did away with the Maynard primer system action - a major improvement for the weapon.
Ammunition was supplied as a paper cartridge containing both round and propellant. A percussion cap would be issued separately and all three elements were combined in a practiced action before the weapon could be fired. The development of the Springfield Musket (produced by the Springfield Armory among others) greatly increased the accuracy - though not to the extent that today's rifles offer - of the standard marksman. So much so in fact that artillery formations were now required to fire from positions further back to compensate for the rifles increased range.
The musket featured, for the first time in any arms market, static iron sights capable of engaging targets at a distance of 600 yards. The sights consisted of two folding leaves (marked simply with a "3" and "5"). When the weapon was fired with both leaves down, the weapon was set for a base "100-yard" targeting range. With the 3-leaf raised, the weapon was then set for 300-yard targeting. Consequently, with the second leaf raised along with the first, the weapon was ready to target enemy elements upwards of 500 yards. Naturally, the South was not in possession of capabilities for the developing and production of large quantities of new weaponry, thusly they relied on captured or abandoned .58 Caliber Springfield muskets to arm their ranks while still relying on their British Enfields or whatever other arms could be imported from Europe.
Almost immediately after the war, the US military switched from the old muzzle-loading type weapons to the newer breech-loading rifles, signifying the end of muskets as standard frontline firearms in American history.