You could form to drop the 4.5gr H20 capacity difference, and use the same bullets/powders/barrels and competitive pressures run with 6PPCs.
As a 30BR shows, the magic in this is not the case itself. It's the conditions. The purpose built & viable underbore condition, that could be applied to any cals (given right bullets and powder for each).
But any new design should begin with WHY..
Why does it make sense to do any other than pick up a phone and order turn-key 6PPC?
I got the impression that the op has a rifle with a dedicated br bolt face so that's one reason someone might want to shoot something besides a ppc.
For me, it's as simple as this....I want any advantage I can get..so why do exactly what your competition is doing?
You have to test things to know if something can be improved upon. I enjoy that aspect of the game and I truly feel that there are better options than a ppc or a 30br.
I've already mentioned that I've had many of both and that I shoot a 30 based on a Grendel because I think it gives me a better chance of winning vs a 30 br. This isn't conjecture but rather, I've extensively tested and compared the two. My conclusion was and is that the 30 Major (Grendel) is best. Not by much...but better.
I campaigned a 6 Grendel this year in a 12 twist shooting 80s at 3300fps. So far, I believe that it leaves nothing on the table at 100 yards vs the ppc, and at 200+, it's appears to be what I'd call considerably better than a ppc. It has other benefits vs a ppc as well, like no fire forming. Just neck it from 6.5 to 6mm, turn and burn.
Again, this is not conjecture but what testing has shown me. I won several matches with it this season against lots of ppc's.
Larry Moore campaigned a 12 twist 6Bra. He won the nationals and I believe has the points wrapped up as well, while pretty much kicking our tails with it and his own 80 grain bullets all season long. This is in UBR unlimited class. Before anyone dismisses his accomplishments for not being group matches, try it first. It's a tough game and has some of the best shooters anywhere, in any br game, be it group or score. Is the bra better than a Grendel or a ppc? I don't know yet...but what he did this year with his was impressive. And fwiw, most are shooting a ppc in this game as well. So, it's head to head and winning regularly against what most will tell you are better cartridges for absolute accuracy. Moral of that story is that just because most will parrot the ppc and 30br to be kings of their respective hills, there are people testing and winning with something else, rather than simply going with the flow.
BR is a competition, just like racing. It should encourage people to look for both large and small gains wherever they can be found..It should not be the BR version of nascars defunct IROC series.
Maybe I'm the only one that feels this way but if my goal is to beat you, I don't think following your every move is the best way forward. Rather, it's stagnation.