I'm late to this discussion and so probably will not be read by many here, but...
Too many historians and history buffs study the Battle of the Little Bighorn or The Battle of the Greasy Grass as the winners call it, strictly from the perspective of the later written histories and military records of the event. Some also include the Indian accounts, which are few.
The military records were probably doctored to cover up an embarrasing defeat. The Indian "records" [mostly verbal] were probably disguised so as to not suffer retribution. It is an oft-stated report that the Indians did not know they were facing Custer. They DID know it. Many of the Indian combatants that day had seen and talked to Custer face-to-face over a period of several years. Several had verbally threatened him face-to-face months before the final battle at the reservation before they left it.
To learn about western Indian/U.S. warfare one must study the West in greater detail.
An Indian with a bow at age 12 could fire arrows faster and more acccurately inside of 30 yards better than most men with a revolver.
The earliest accounts of fights detail not "running for the timber" as many people thought, but in "running for the open ground with a wide firing lane". As in buffalo wallows, high clear hills, or down behind the horses in very open country. In such instances the single shot "Mountain Man Gun" and any other firearm was superior to larger numbers of Indian warriors. Up to and beyond Bighorn/Greasy Grass bows and arrows were still used in profusion. Firearms, even single shots could, and did have telling effect against greater odds. A case in point is the Second Battle of Adobe Walls. 24 men stood off 500 to 700 Comanche, Cheyenne, and Kiowa for about four days. The defenders were inside of buildings and the attackers were out in the open.
Custer provided the U.S. Army with its most successful tactic against the Indians at Washita. He attacked the village in the dead of winter knowing that the Indian people would be there and not be prepared for his assault. His intention was to capture as many women, children and old people as he could. He suspected, or knew, that the warriors would fight a skirmishing retreat and as they did so the "non-combatants" were to flee as do quail on open ground, only later would they regroup elsewhere and supposedly safe.
At Washita, Custer captured a 100 or so non-combatants, destroyed as much camp gear and equipment as he could and took or destroyed the most important Plains Indian "tool"...the horses. He also realized the village was larger than thought and he did indeed leave some men to die. Questionable conduct, but not the first time in war men [and now women] were "sacrificed for the greater good" [The Phillipines in 1942 as an example.]
After Custer committed Washita the warriors who fled had no families, equipment, or horses. They later turned themselves into the agencies. And yes, many stayed out.
After Washita, it became standard Army practice to find and attack the villages. It became standard practice to split the command to capture as many non-combatants as possible. Overly large numbers of Indian warriors were not feared. They were expected to fight a skirmishing retreat and then disappear themselves. And the Army weapons were superior to anything the Indians had at all but the shortest ranges, even the very few repeating rifles before Bighorn/Greasy Grass.
Yes, the Trapdoor of 1873/1876 had its shortcomings and it did have its ammunition failures but it was still more reliable than reported today. And it was more accurate and effective a much longer ranges than the .44 Henry and the .44-40.
After capturing the prisoners, the village and all its contents were destroyed, usually along with the horses.
It is known and published fact the Army was always outnumbered by the Indians in several pitched battles. They were perhaps in fear of their fate but they prevailed due to superior firepower, open ground, and cover. The Battle of Beecher's Island is a good example. Fifty men against an estimated 700.
So on June 25, 1876 Custer did not consider himself outnumbered.
The Gatling guns were worthless. They fouled quickly and would fail to operate. There were cumbersome and slow to drag through the terrain and any good cavalry officer who needed speed would have left them behind. In the final battle at Bighorn/Greasy Grass they would have not been able to unlimber them and use them effectively. And...the Indians would not have stood around and let themselves be mowed down by Gatlings anyhow. Very early in the fight the Indians "went to ground" and used to coulees and brush to stay below the line of fire. Under cover they could launch their arrows in deadly arcs and at close range the firearms they had [not as many as suspected] could be used more effectively. It was a case of the Indians in trenches and the Army in the open. Essentially by happenstance the roles had been reversed. The Indians were under cover and the Army was not.
Custer was very knowlegeable about the Western Indian cultures. He knew he was on the track of a large village. He knew he was seeing a large village. BUT, he was counting on the usual skirmishing retreat. He was not expecting a frontal assault on his assault. In fighting of any kind the mindset of "retreat" is different than the one of "assault".
And, Custer was unaware that Crook had been defeated at the Rosebud only about two weeks before by the same Indians, using the same tactic, assaulting the "assaultees". Had he known this he might have acted differently.
Custer was under orders to do just exactly as he did. Find and "fix" the enemy. If he could wait until the other forces came up he was to do so, BUT if he thought they were to run he was free to attack. When he saw the dust cloud of the pony herd he thought the Indians were gathering up the horses to flee. They were not, they were gathering them up to attack. Hence he acted on the prevailing tactic of the day for the Army, split the command to capture as many "non-combatants" as possible, send in a direct assault [Reno] to cause/create the retreating skirmish, and to not care about killing/capturing warriors, as they would eventually flee. Once the non-combatants were captured and headed back to the reservations the warriors would follow because the were extremely tied to family and community. Plus they would have no resources since they would have been destroyed by the supposedly victorious Army.
Custer saw that his plan was awry when he saw Reno's retreat [he did]. That was when he recalled Benteen and he himself tried to attack the village, either to take the pressure off of Reno or to try a second attempt at causing the "skirmishing retreat". It failed because the Indians weren't playing according to schedule.
It is very suspected that Custer was mortally wounded at the attempted creek crossing and whoever was in command then began the final retreat. Probably attempting to seek open ground where the longer range and more accurate Trapdoors would have had telling effect. They never made it. They threw out skirmishers and eventually dismounted to fight beside their horses, in cover, as had been successful in the past in many, many engagements. But it was too late. The warriors in the coulees had the advantage, they had cover and close range.
Note that the Indians did besiege Reno/Benteen but never pressed the attack home. Open ground, entrenched troops, and excellent long-range single shot rifles [the Trapdoor] kept them at bay. Also it was still an ethic of the Indians to avoid large numbers of casualties. So they broke off the fight, just as is done in almost all guerilla wars. They could not afford the casualties, that is why the fought so few pitched battles. However at Rosebud, Bighorn/Greasy Grass, and even later with the Nez Perce in 1877 the American Indians were in full realization that their backs were to the wall, they had to now stand and fight.
Custer's Crow scouts did not "run away", they were ordered away. Probably for two reasons. All Indian cultures were not accustomed to taking large numbers of casualties therefore they rarely attempted combat against larger numbers of warriors. Custer knew his scouts had done their duty, he knew they were not willing to fight larger numbers, therefore those that he could he sent away.
As to Custer ignoring the scout's warnings of the size of the village. Custer knew what he was seeing. I suspect he expected the Crow to be nervous of such an attempt. They did not want to fight overwhelming numbers, so he deliberately "downplayed" their concerns, when in actuality he was seeing what they were seeing and or believing what they were saying.
That Custer was a huge egotist is inarguable. But then again a lot of military men are. But a true egotist is not going to foolishly expend his command and therefore risk failure. He will usually act the opposite and hoard his troops and resources until perhaps too late to be effective as he doesn't want to lose.
Custer was a man full of "vim and vinegar". He rode more spirited horses than most, he rode harder and longer than most, he stayed up longer hours than most [reading, writing, practicing taxidermy]. He had mistresses it is rumored, he swallowed life in huge gulps. Not because he wanted to be better than anyone else but because he didn't want to miss out on the various facets life has to offer.
His failing grades at West Point were probably an indication of a spirited young man who rebeled at convention and rule because it interefered with tasting life's joys [to him]. Also bookwork rarely sets well with the individual who is constantly wanting to be active, or athletic. Ask any high school teacher. Some of their most "problematical" students are the jocks, they don't want to deal with the boredom of school, they want to be in the game, they even hate practice, they just want to play. Custer wanted to play so much he failed classes and tests. But he was not without intellect.
Essentially, as I have come to believe, Custer was following orders and the conventional tactics of the day. The Sioux, Cheyenne, and Arapaho did not. They had changed their tactics at Rosebud and again at Bighorn/Greasy Grass. They had to. No longer could they retreat as they had nowhere to go anymore [in their mind].
Custer tried to adapt to the change but was unable to in a timely manner and his supposed mortal wounds changed things also.
I do believe Benteen [by accident or intent] failed to follow Custer's direct orders. He dawdled in his approach to Custer and ran instead into Reno. He had officers demanding he move forward. He refused. Finally one officer was granted permission to move forward with his small detachment [Captain Wier I think] and he ran headlong into the warriors who had just defeated Custer. He had to retreat back to Benteen and Reno.
But, had Benteen bypassed Reno and attempted to relieve Custer I think he too would have been swallowed up. Had he successfully joined Custer he definitely would have. The terrain favored the warriors. Had he found some open ground with cover before he got to Custer and the Indians, he might have survived, or portions of his command at least.
There were also reports from the Indians [and the women] that yes, women took arms and attacked with the warriors, they too knew that this time they had to stand and fight. So the story of the woman who actually killed Custer [if he was not already dead] might well be somewhat close to the truth.
Essentially a properly planned attack [according to the thinking of the day] was at first blunted and then overwhelmed due to favorable terrain and a changed mindset in warfare. They Indians were attacking and not retreating. Custer was aware of the change but could not adjust in time. His time had run out.
