If you read the threads in SSS, you'll find the Army's rationale for using a hard-hitting single-shot.
That said, nothing can beat thorough training, and in order to do that, you have to be able to back it up with funding for ammunition, amongst other things - something that the frontier Army just didn't have.
Custer's Seventh - like every other Frontier Army outfit - was filled with newly-arrived immigrants and men trying to get to the gold fields while eating Army rations.
They weren't elite, well-trained, highly-motivated, agile, mobile, and hostile troopers by any stretch of the imagination.
Custer was conventional Cavalry combat-experienced, and an experienced leader of men - ask the Michigan Wolverines - but he was also hungry for glory and a chance to redeem himself - and he - like everyone else - seriously underestimated his foe's capabilities.
Never before - and never since - would American troops encounter the number of Indians in one place and cocked, locked and ready to rock - and they reminded the Army that there were still lessons to be learned on the modern battlefield - lessons we would take to heart.
It would take Crook's implacable Infantry to bring the Indian Wars to a close - but there was no way John Ford could add 'romance' to hard-campaigning dogfaces who pursued in winter, so that part of the truth is always left to the dust of history.
Scouts Out!