Pattern 1853 Enfield Rifle-musket

Both sides imported large quantities of European firearms to assist them in arming their troops. The most widely used was the British Pattern 1853 Enfield rifle-musket because it was accurate and well made. An important factor for both sides was that the .58 caliber bullet used by both Union and Confederate forces was interchangeable with the .577 Enfield. Hundreds of thousands of Enfield rifle-muskets were successfully smuggled into the South by the Confederacy during the war.
(from Civil War @ Smithsonian)

Enfields were made at the Enfield factory, the London Armoury Company, the English Gun Trade, and the factories of Liege, St.Etienne and in Windsor, Vermont. About half a million were purchased by the Federals and a very large (but unknown) number by the Confederates.
(from Managing The Enfield, by WS Curtis)

Very common in many Federal and most Confederate Infantry units.

.577 or .58 caliber bullet (minie ball)

0.577 inches

1000 yards

Overall: 55 inches; Barrel: 39 inches

9 pounds

See a fine period (1859) article about the making of the Enfield.