As many of you know, I come from the world of handgun silhouette where no one shoots standard cartridges and often loads have to be invented. Done a bunch of that myself. It can get scary. Its usually safer, much safer, to stick with what has been previously tested when and if available.
Different powders, though of the same burn rate, have different temperature rises, different pressure rises, different pressure tolerances. That’s why the old SR4759, which works great in light or reduced loads in large cases but using WW 296, if it would ignite might get splashy. Both have identical burn rates.
Point, might need to look at a full ladder test, try different primers. My TCU loved BR4’s, my fast twist .221 Fireball shooting 80 grain Bergers loved WSR’s and neither handgun would shoot well with the opposite brand of primers).
And now, the final point. Some rifles don’t shoot certain bullets well. My guess would be the bearing surface, and where the bearing surface interacts with the crown when leaving the barrel. I had an XP100 in 7BR. Using a 140 grain Sierra Matchking, it would be on target at 100 meters. Using a 145 grain Speer HPBT, it would be 7 inches to the right. (All other factors were equal) Its a crazy game and getting there is all the fun (till it gets frustrating)
And I agree with others, just because you have a bunch of powder, does not mean you should use it. Even today, poweder is cheap; barrels, actions, eyes, arms…not so much.
