Absolutely right on the Indians' use of repeaters. What Fox found, however, was that the Indians' repeater ammo was mingled right amongst the cavalry---not 50 yards away, as if the soldiers were shot down from range, but as if the enemy had used the grass and ravines to conceal their approach until they were right on top of them.
Also, as you well know, and has been documented many places, the cavalry's carbines were notorious for jamming.
There were two volleys by the Sioux/Cheyenne into the 7th from behind the military crest of the hill. This is incidentally the same tactic that Wellington used against Napleon's Old Guard to such devastating effect. Almost no unit can take those kind of losses and pick up the pieces. The 7th was doomed within seconds of first contact.
There is consderable archeological as well as narative evidence to this effect. The rounds found closer among the troopers are there since the Indians held the discipline of a firing line for only the few seconds to fire the two volleys then resumed their normal warrior style fighting.