Model 1861 Percussion Rifle-Musket

The Model 1861 was the standard rifle-musket used throughout the war. A single-shot, muzzle-loading gun detonated with a percussion cap, it was originally made by the Springfield Armory, but due to the need for more firearms, the U.S. government contracted with twenty private contractors. It is estimated that about one million Model 1861-type muskets were manufactured during the war. It was a modification of the Model 1855 rifle-musket that eliminated the patch box and the Maynard tape primer. These changes lowered its manufacturing costs and time, both of which were beneficial to a country at war and in need of firearms.
(from Civil War @ Smithsonian)

This was the most common weapon in most Federal Infantry units at Antietam. It was also in wide use in Confederate units following capture and battlefield scavenging. A large number of these were captured at Harpers Ferry immediately before the battle.

.58 caliber bullet (minie ball)

Maximum: 500 yards; Effective: 300 yards