Throughout the Civil War, the Union infantrymen were usually better armed than their Rebel opponents. Antietam was no exception. The most common shoulder arm of the Yankee foot soldier was the Springfield rifle. This does not mean that there was not some degree of diversity of arms in the Union ranks. For example, some units such as the 7th West Virginia were armed with the British-made Enfield rifled musket. The 20th New York carried the U.S. Model 1841 Mississippi rifle with saber bayonet. The New York regiments of the Irish Brigade were issued the Model 1842 .69-caliber smoothbore musket. This was actually a favored weapon with the commander of the brigade, since it could fire “buck and ball” (a load of buckshot and musket ball) at close range with deadly effect.
Correspondence sent from an ordnance officer in the Army of the Potomac to the chief of ordnance in Washington several weeks after the battle indicates that five thousand smoothbore muskets were still being carried by elements of McClellan’s army. Esteemed Civil War weapons authority Joseph Bilby has stated that “as late as the battle of Gettysburg, July 1, 2 and 3, 1863, 10.5% of the regiments in the Army of the Potomac, the best equipped Federal army, were still armed, in whole or part, with obsolete smoothbore muskets.” Bilby goes on to say that “except for their percussion ignition, these guns differed little in ballistic capability from the weapons shouldered by those Yankee soldiers’ grandfathers in the Revolution and the War of 1812.
from “Battle of Antietam: The Bloodiest Day” by Ted Alexander