High power, Prvi makes brass and bullets for 8x56r. There is even GC cast lead out there.
JTJ, I'll explain the 7.5, .308 deal again. The K31 7.5x55, M36 Mas 7.5x54 both use .306/.308 bullets, they always have. You are splitting hairs over 2/1000th's of and inch. A 7.5 bore is .298.. .300 at the lands and .308 at the groves.
For the power factor I'll use FPS as the higher the velocity w/ a specific powder, the higher the pressure. Case Volume on .308, 7.62x51, 7.5x55, and 7.5x54 are very close, but the action strength dictates maximum velocity and pressure.
150gr bullet for all using IMR4064. These are actual chronograph velocity at the high end of powder charge for bolt guns.
.308: 2800- fps
7.62x51: 2720- fps
7.5x55: 2620- fps
7.5x54: 2680- fps
GP11 ammo was developed in 1911. It was a milestone in its day. GP90 for the 1899 is a different animal, as is pre jacketed spitzer 8x57. When jacketing bullets European countries tended to make them larger if they were RN lead first.
Sorry, I get real worked up on "theory". The load difference w/ these cartridges is less than 4%. The high end for .308 is 46gr.
These metric calibers are what I do. I don't own a .30-06 or .270. I have a 8x57 and 6.5x55 instead.
With a 6.5x55 w/ a 120gr at 3000fps or a 129 at 2900fps I'm not losing to a bit to a .270. Plus my rifles are true pieces of extreme craftsmanship. My newest is from 1962. Most are early to mid 20th century including my K31. BTW, my K31 will outshoot my standard M96's and M38 Swede's, that says allot.
I apologize if I jumped down your throat before. But, as you see I am very passionate about my Metrics.
BTW, an 8x57 w/ a 150gr at 2900fps+ is my norm.