It can be arguably said that the Spencer repeating
rifle was a revolutionary military
weapon and one of the greatest
force multipliers in history. Prior to its appearance on the
battlefield, a soldier with a
muzzle-loading weapon could only fire about two or three times a minute. Even a
breech-loading single-shot rifle could only fire about once every few seconds. With the lever-action Spencer, a
soldier could fire about once a second or better. In one of the first battles to see the weapon used, at
Hoovers Gap on June 24th '63, the
Confederate officers thought that the
Union soldiers were being
reinforced by fresh troops, such was the volume of their fire.
The rifle fired rimfire bullets, and held a 7-shot tubular magazine in the stock, inserted through the butt plate. A multiple-tube carrying box was developed for cavalry, who found them extremely useful. This also sped the transition of cavalry from scouts and skirmishers to mounted infantry.
It also had a part in the movie Unforgiven starring Clint Eastwood. He and Morgan Freeman's character use it in a pivotal sequence.
Sources:
http://www.civilwarguns.com/spencer2.html
http://www.romanorifle.com/SPENCER_Firearms/spencer_firearms.html