Of course we need to remember that this particular device was not patented until Dec '64, so it would have seen very limited use in the war. Most of the reloading would have been one at a time and the troopers were not issued rounds in great number either. The arsenal packs hold 42 with 7 rounds individually wrapped and 6 of those in a pack.
Major-General James H. Wilson, who was instrumental in crushing Hood at Nashville (15-16 Dec. 1864) and defeated Forrest at Selma (2 April 1865), wrote the following about them: "There is no doubt that the Spencer carbine is the best fire-arm yet put into the hands of the soldier, both for economy of ammunition and maximum effect, physical and moral. Our best officers estimate one man armed with it [is] equivalent to three with any other arm. I have never seen anything else like the confidence inspired by it in the regiments or brigades which have it. A common belief amongst them is if their flanks are covered they can go anywhere. I have seen a large number of dismounted charges made with them against cavalry, infantry, and breast-works, and never knew one to fail.
Having said that, remember, when Wilson went to Selma, the troopers carried limited suppllies... 'Each trooper carried 5 days light rations, one pair of horseshoes and 100 rounds of ammunition on his saddle. We also had a supply train of 250 wagons, carrying 45 days rations of coffee, 20 of sugar, 15 of salt, and 80 roiunds of ammunition, besides 5 days rationss of hard bread and 10 of sugar and salt on pack animals". That's only 180 rounds for a major engagement.
Sgt Scott