The Spencer Rifle was the customary firearm for the Union cavalry trooper. The carbine version was used because it was shorter, lighter, and easier to reload quickly. This weapon was like because it was easy to load and relatively accurate when shot from any position, prone or horseback.
The Union army was at first reluctant to buy the firearm because it would require a drastic change in supply in order to provide the much needed ammunition. This rifle was a repeater so it was able to hold 7 rounds in its magazine tube. A soldier could fire roughly 20 rounds per minute. It therefore required a lot of ammunition.
The Spencer was the most advanced firearm of the day. The action of the rifle and the firepower was devastating. There were some drawbacks though, firstly, the cloud of smoke that was to be released could be a strong hindrance for aiming. It is difficult to see through a dense cloud of smoke and coupled with the issue was that the number of men firing at the same time made it very hard to see.
The second major issue that confronted the troops that used the Spencer rifle was once again, the ammunition supply problem.
There were just not enough of the necessary cartridges to adequately support a division let alone an army. The usual course of the men who used the Spencer during a battle would be relegation to the sidelines. Popping off the occasional shot as the armies fought in the interior of the foray. The problem most affected the Confederates who were lucky enough to capture one of these rifles.
Once they ran out of whatever ammunition they also captured they would no longer be able to use the weapon. The south could never supply the needed copper in order to create the cartridge casings needed for the ammunition.
The Spencer was a high quality rifle that could easily turn the tide of any battle where the opponent was still using outdated single shot muzzle loading rifles.