As I noted, I'm away from my books this week and won't be able to delve into this until at least this weekend.
But, P.O. Ackley did some testing with dry and oiled cases and recounted the results in his books using a lever-action rifle with excessive headspace.
He found that dry, the primer was backed out, the result of the case gripping the walls of the chamber.
When the cases were oiled, the primer was reseated in the case by the reward slip of the case AND the cases separated. (thanks for the info, Mal!)
"I figure that the Japanese machine guns had oilers because they did not copy the French Hotchkiss closely enough to include primary extraction."
Exactly, which is what I noted earlier. Lacking primary extraction, the guns would rip the heads off the cases during extraction.
The Italians, I believe with one of the Breda models, attempted to get away from the oiled cases by fluting the chambers so gas pressure would "float" the cases out during extraction, but it wasn't enough and they had to go back to oiled cases.
Once again, though, this scenario transpired only with automatic weapons.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.