I agree that in a perfect chamber, with perfect brass, that the bulk of the cartridge will be an "unsupported tube."
What effect does gravity have on an unsupported tube?
Think putting the round in the chamber by hand, then using a bolt with no extractor or ejector on the bolt face, where is gravity creating stress in the brass?
There is no contact with the bolt face to add support to the case head, so my original summation of perfect brass, perfect chamber, no lateral pressure stands.
Sorry if I didn't make that clearer.
Machine guns are awesome until you have to carry one.