Obviously it's startling to hear of such pressure from a minimal load of varget, but despite extenuating circumstances like the thinner cup of a regular CCI #400 primer and seating to touch the lands it's just as obvious to me that the larger issue here is prudent powder selection when working with a barrel that's significantly longer than a 24" test barrel.
With all due respect, seating a few thousandths off the lands isn't going to be especially helpful to the young new owner of a savage.223 target rifle with a 30" bbl who's made an imprudent propellant selection. Published load data is extremely valuable, and a lot of work and sweat has gone into these tables, but I found out the hard way that when you get into over-length barrels all bets are off - some of these propellants can really pack a wallop if they're given enough running room. There are those of us who still use old fashioned long rifles and FWIW I just hope we're not obscuring the very real issues associated with reloading for them.
Your original post indicated you had a blow primer and mushroomed case. From the get go you seem to want it to be the "powder"
There are several, if not many other things it can be, but you want to belive it is "the powder" or the barrel length.
You seem reluctant to look at any other possible cause, you will not indicate trim length or COAL. So there is not much others can help with other than speculation.
Hope you figure it out.
As your signature states:
"Theories require facts, Captain."