If I'm reading your question right, you are assuming that the 250s will hit higher than the 300s with the same powder charge....

First, you are going to have to shoot the gun to see what is happening....But I would bet that the 250s actually
hit LOWER than the 300s, with the same powder charge and no change in scope adjustment....

I have seen this in pistols and muzzleloaders...Here is why I think it happens...

A 300 grain bullet spends more time in the barrel than a 250, so recoil has more time to effect the bullet, sending the heavier bullet higher than the lighter bullet...In other words, recoil kicks the bullet higher...This isn't usually noticable in a centerfire because velocities are higher...