It must also be noted that a military round of the time, needs to be able to kill a horse, and do it fairly rapidly. From the front. The .44 Henry RF will not penatrate the front muscle wall of a horse, when holding center on a charging horse and man. This was a problem with all the pistol caliber rifles/carbines of the time.
Bufords men were armed with Sharps carbines at Gettysburg, BTW. The only ones carrying Spencers there were the Michigan brigade under Custer, which did splendid work on the 3rd day. But the confederate advance on the first day was delayed by Sharps Carbines.
We in the GAF have also found the Winchester style repeaters to be at a disadvantage overall in skirmish work. The slow reload really sets them back. Often a well run single shot can outpace them in a prolonged firing sequence with mulitiple targets and movement.