At the time of Little Big horn on the plains the loud boom of the cavalry carbine could be distinguished at distance over half a mile. This was linked to comment by Reno and Benteen commands, that they could hear rapid shooting, then sporadic. A pause as both ides re -grouped. Then the same over again. Would have been disconcerting to hear. Not being able to know what the situation was, or to go to aid those men.
Last Stand Hill is a few miles from Reno Hill. Those heard sounds were probably reported by those who advanced to Weir Point.
Walked a good portion of Little Big Horn/Greasy Grass battlefield a few years ago - the gully below Last Stand Hill still harbors the remains of quite a few of the Cavalrymen who lost their lives.