It is alleged that the shell casings are all copper except for one casing believed to be from Custer’s Remington .50-.70 caliber “rolling block” sporting rifle.
https://www.antiquesandthearts.com/custer-collector-michael-ward-unloads-at-heritage/
From the above article: "Keith McDougal Jr, said he found it at a known Indian position, which invites theories as to whether the rifle was scavenged and taken by the Native Americans in battle. He wrote, “I personally have seen more than 2,000 shells found at the Custer Battle in the last 15 years and no other shell casings of different calibers have been found which were made of brass. All other shell casings were made of copper."
The copper casings allegedly caused extraction problems when the chambers became too hot. These needed to be pried out, which had to be time consuming and frustrating, especially if you're under attack. Although, like so many other things about LBH, the number of failures of the copper casings has been disputed.
FWIW, when I fire many rounds through my modern weapons and I'm using steel cased cartridges and don't stop to let the barrel cool I've had a couple of occassions with the same issue of the casing not ejecting after firing and needing to be pried out. Think there's possibly some kind of lacquer on them.