I second the OCW method. For the loads I've used it, its made for loads that can deal with variations I may put into the batch. This is only for the guns I've really wanted to spend time on tweeking the accuracy. Much of my reloading, I'm just doing milsurp calibers for the fun of it.
I"m no wise old expert but going from 1.5" to 10" with just a primer change does not sound right though. If you have a good bullet, you should not vary that much unless there is something Really inconsistent about what you are doing or how you are shooting.
Scoped? something loose?
Also watching this thread with interest. Not at all discounting what OP notes here, as his target showed what it showed. But unless that chronograph shows data that's for some reason all over the map, and erratic to a point that would explain group size growth by a factor of 6 or 7? I also think there's likely something else going on here.