Re: Help! Extreme spread and standard deviation loads
I address powder first, then seating depth, all the while paying attention to brass prep including case length.
I usually run a ladder test in .5g increments with the bullet seated .010" off the lands and try to find a couple of nodes with good accuracy while checking for signs of pressure.
After the first ladder test, I try some loads at smaller powder increments around one or two of the nodes that gave good accuracy. Around one of these nodes, the grouping and ES/SD should be less sensitive to +/- .1g of powder.
Then, I take the best of those results and start varying seating depth which will also affect results. Meanwhile, you also need to pay close attention to consistent case length and brass prep by this time. Lack of attention/consistency with any of those variables will affect your ES/SD for your MV.
Hope this helps.