Another thought. If the case shape and capacity to bore ratio override everything else, then the old Soviet 7.62X39mm M43 ought to be broadly on a par with the .30BR. Being mostly chambered in SKS and AK series assault rifles whose internals rattle if the rifle is shaken doesn't make for super-precision of course, but one doesn't hear of many (any?) super-accurate bolt rifles in the chambering either.
I had one some years back in factory form, the Cz527 Carbine with Mini-Mauser action and 16-inch barrel. With Lapua factory ammo and the best of my handloads it would group under an inch at 100, the rare fluke group down at half that. I was much amused by people who'd say to me on the range words to the effect of .... "It'll be a good shooter I'll bet. The PPC is based on this cartridge." It didn't stay long as whilst a great plinker, we have few to nil opportunities in the UK for off-range plinking, and it wasn't suitable for any form of range based competition I indulge myself in.
It can be made to shoot better I'm pretty sure, but large primer (Remington brass aside), large flash-hole, and brass quality likely make it impossible to ever match the PPC or 30BR its nearest equivalent. Glen Zediker mentions in his original book
The Accurate AR-15 having used a .22PPC in XTC at one stage, but to keep brass costs manageable, reformed commercial 7.62X39 cases to make it .... and it seemed to work, but a huge cost in time and effort. Anyway, XTC and BR make slightly

different demands on rifle, ammunition, and shooter!