Re: New Cooper 6.5x284
He could get 2950fps at 18degs with 56.8gr 4831SC. This is ~2Kpsi above SAAMI max, and with a muzzle pressure of ~12.6Kpsi(very high). At 70deg the same load will likely put him at 3030fps, 8Kpsi over SAAMI max, same muzzle pressure.
53gr 4831sc will only get him 2830fps at 70deg.
He won't even get close to 4831 velocities using Retumbo.
The highest there is ~2800fps with a case full(including a full fireformed case).
His pressure would be 15Kpsi below SAAMI max, and likely producing very high ES.
The best he can do velocity-wise in 24", and possibly overall, is 60.5gr Ramshot Magnum at 70deg. This is barely SAAMI max, ~3016fps, not compressed, but muzzle pressure is still high at 13.4Kpsi.
With this load same conditions and a 28" barrel, he'd have 3130fps, same pressure but muzzle pressure would be down to where it is right now (~11Kpsi).
This is what a 6.5x284 cartridge is for.
No matter what, his muzzle pressures are high with a 24" barrel & this amplifies any flaw in release of boat tail bullets. It may not show up at 100yds, but it's a factor in distant shooting. The slightest crown flaw will show up big time, and ES is raised also.
It never made any sense to run big cartridges with short barrels..
With a 28" barrel, my 6.5WSSM can run circles around a 6.5x284 in 24". But a 6.5x284 in 30" would leave me in the dust(as you would expect from a way bigger case).
His gun, with that barrel length, would be perfect in 6.5x47L.