There is a factual reason for that observation, and it is powder ignition consistency.
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I got decent accuracy from my 300 Weatherby magnum using 2400.
Try 1 gr. of Dacron to hold the powder on the primer.
About 15-20 years ago I bought a custom barreled Ruger #1 in 32-40. It came with about 40 rounds or so for ready loaded ammo that were recorded as having 16 point something grains of H 4227 behind a 200 gr bullet and a Federal Match primer, IIRC. Great performance in all, in fact probably about the best fixed ammo in 32-40 I ever shot. I don’t think primers are your problem.
Froggie
7mm Mag is a pretty big case to be using 2400 or 4227, IMHO.
If I *had* to use something at less than about 80% of case capacity, it would be 700X or Unique. Too many powders are supposedly good for small amounts in big cases, but produce weird results if not helped to the back of the case. I don’t like tipping the rifle to make sure no weirdness happens, but then, I’m still a fan of WC860,870,872 for anything .303 or bigger. And Jeff still has WC870 at $7.50 a pound.
I agree with Froggie, it’s not the primer,,,,