I haven't followed your load workup, but the biggest error most new reloaders make is to try too many variables and not do a methodical job. ...often jumping to conclusions because they had one tiny 3 shot group.
It almost sounds to me like you need to scrap everything and start over with Dan's OCW method.
I see no reason why h4831sc won't work. But, pick some known good set of components (e.g. 142 smk, lapua brass, h4831sc, cci br2 --or-- 140 berger, norma brass, h4350, fed 210m) and stick with the method.
I also might add that I rarely find best accuracy with max loads. It almost sounds like you've pre-suppossed a velocity and are trying to get accuracy at your max load.
The 140/142's are still very lethal at long range with an MV in the 2800's. If that's not ample, then you may want to consider a 7mm cartridge.
You seem confident in your shooting ability, so I would assume that technique, paralax, scope, mounts, rest, etc... are not at issue here. But, that's not always the case.
After 200+ rounds, do you have a specific load that you can say is consistently sub-MOA to 300+ yds?
-- richard
When I first started my original goal was 1000 foot pounds of energy at 1000 yards. Given all the different variables that put me at a velocity around 2950 fps.
My goal from a factory rifle was .5 MOA. I feel that is achievable.