Although the Spencer rifle had been developed as early as 1859, it was not initially used by the Union. On August 18, 1863, Christopher Spencer walked into the White House carrying one of his rifles and a supply of cartridges. He walked past the sentries, and into Abraham Lincoln's office. After some discussion, he returned the following afternoon, when Spencer and Lincoln were joined by Edwin Stanton, Secretary of War and other officials, and the group then proceeded to walk out on the Mall. Near the site of the Washington Monument, they engaged in target shooting.Okay, after I digested that, here's the history of the Spencer. Tube magazine fed, it was a leap forward in firearms. Capable of sustained firing, the Spencer rifle so far outdid the muzzle loaders it was replacing that the tactics of the day didn't figure it in.
LoFan John also mentions a replica, so he'd have a shooter to go with his grail gun. So here's a video of a replica (in the carbine size) being fired and the parts and functionality explained. The rate of fire is impressive.
Looks like a nice original can be had for $5000 and a replica for about $1200. A pair of rifles to fulfill both a collector and a shooter.