IMR4895's not normally used for 7 Rem Mag's with bullets heavier than 110 grains. IMR4350 or IMR4831 is much better for 150's. In any case, you need to record the accuracy of each load separatly so each load can be compared realistically to the others. But if all 21 went into 5 inches at 500 yards, that's pretty good. Shoot 5-shot groups with each load then compare them.
Others posting their group sizes without mention of the range they were shot at probably did so at 100 yards. Their load's accuracy at 300 will typically be 4 or maybe 5 times as large at 300 yards.
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