Until very recently, throughout most of american history, civilians, even indians, were better armed than the common soldier.
My own family commonly used and carried revolvers and repeating rifles decades before the common american army soldier routinely carried them, or was issued them. Any civilian could buy and carry any machine gun until 1934. Revolvers and repeating rifles were usually purchased by civilians and indians, not the US Army, until the 1900’s.
Custer could have brought along Gattling guns and cannon, but he felt it would have slowed him down. He, unfortunately, did not have a choice in dediciding to have his men carry the repeating rifles Winchester 73’s or Henry 66’s, it was the armies choice. Indians, on the other hand, could, and did, buy repeating rifles.
The indians did not have/choose to use Gattling guns, nor cannon, therefore bringing along Gattling guns would have made up a great difference in making up the shortcomings of the US Army being “underarmed” in any battle at that time.
Custer was never ever known for being a “smart” man, he was lucky. Custer rose to fame and rank thru luck, not intelligence. His luck ran out in 1776.