Have you kept the barnes targets you shot? The shape of the holes can be quite diagnostic. If not shoot one at a target at 100 then one at another at 150 or 200 and look at the holes. If they are not perfectly round but oval or show a nice bullet profile at either distance then all the buggering round with seating depth or powders will be of no use as the bullets are too long for the twist of the rifling to impart gyroscopic stability to.
In some cases pushing the speed up does give a window of stability but as soon as the speed drops it all goes bugger up again.
By the way what is the twist rate of the rifle.
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