"Yes, but copper is fine for holding the primer initially. It was when the army experimented with reloading that the issue manifested itself."
Actually there were problems before that, as well. Apparently the copper cases, once formed and pierced for a primer, would rather rapidly crack around the primer pocket and during shipping the primers would actually fall out. Annealing, I guess, wasn't thought of for cases back then.
From what I understand that's why commercial manufacturers adopted brass for case manufacture LONG before the military did.
"I've even got one cartridge with no primer whatsoever; just a little hole of about less than one millimeter in the center of the base. Still has the bullet, too. I think it's a Maynard. Really wide rim, about a .50 caliber bullet."
I've got one of those.
Externally primed cartridges were a transitional step between paper cartridges for muzzle loaders and large-bore self contained center-fire cartridges.
Their heyday came during the Civil War. Maynard, Smith, Gallagher, and Burnsides carbines were the big names in externally primed cartridge carbines.
Along with the Maynard, I also have a Gallagher and a Burnsides cartridge.
In fact, I owe someone photographs of all three...