Your experience is apparently an example of what Tommie is referring to above with respect to newer powders available.Some wierdos like 270 cal.
I have/had 270 wsm, 270wby, soon a 27 Nosler. The worst was 270AI, but I'm blaming Douglass! Wanted a 9 twist, they made a 10 twist. Not only that, the headspace was long, had to jam the bullets to fire form. I saw 100 fps more than the standard 270win, never found a consistent load after exhaustive load development.
By far my favorite .277 cal as of the last few years is 270wsm. The performance of RL26 in this cartridge is astounding!
My favorite Ackley is 6mm Rem AI. 105gr at 3350 for an accuracy load in 26". ( I never maxed out pressure, so not really sure what top gear was). And yes this was with RL26.
Ackley summed up the customer (I.e. not his idea) requested improved .244 Remington as “only a modest increase in velocity is possible by any so-called improvement and this increase is only possible by the use of considerably more powder.”
While he never improved the 6.5x57, he was a fan of the .260 A.A.R., which was an Apex Rifle Company design. He called it a “very fine 6.5 cartridge.” With 6.5’s being the rage I think this would be the cartridge to depose the 6.5 Creedmoor in short order if Americans were willing to loose a little weight.