I would guess the level of sealed environment will vary from case to case and from several obvious factors. If however, a round was loaded and sealed well enough then I could imagine the difference between inside case and outside air pressures having some effect on burn rate. As far as the lack of oxygen goes however; as I understand it, gunpowder propellants are considered oxidizers meaning that they provide their own oxygen for combustion and would ignite within a vacuum. Think of rocket fuel in space. Given the original scenario at hand I'm leaning toward something else being the cause. I gotta say this thread has gone down some rabbit holes! So many theories but I'm kinda feeling like this one might remain a mystery. Been fun to read though.I’ve always thought the case was a sealed environment?