How many off center firing hits have you ever seen.
Even a rifle cartridge with a shoulder doesn't mean the chambers shoulder will lift the rear of the case.
Below a 22-250 fired in a Ruger #1 rifle that does not have a ejector. This happens with small diameter cases and large diameter chambers. So tell us again how the chambers shoulder will lift the base of the case and center the rear of the case in the rear of the chamber.
Your inexperience with different type actions other than what you were told about the M14 is showing.
Below why the rubber o-ring was used to fire form .303 British cases in the Enfield rifle. And the same applies to new belted cases where the case shoulder is short of the chambers shoulder.
Below 45-70 cases bulging at their base, again a small diameter case fired in a large diameter chamber.
I have seen fired cases set on their base that are leaning to one side like the Leaning Tower of Pisa. And this was because the case was laying in the bottom of the chamber when it fired.
And did you cover this below in Navy A school????
Or wonder why Lake City 7.62 cases are thicker in the base and made of harder brass.
P.S. Don't tell F. Guffey about this, he doesn't have a thick enough feeler gauge to fix the datum. 
