Shell deformation when stored.

Currently my only shotgun that's mag fed is my Vepr 12 and that's just a stupid fun range gun. But with the advent of the new mag fed 870s coming out it bears to question if there's any truth to the old tale of long term storage of mag fed shells deforming and causing feeding issues. How true is this? And conversely what about long tern storage of a tube fed shotgun? Any deformation to the shells as well? Just on a different axis?

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