Tuning. From the outside.
The essence of tuning, short version.... the REASON for tuning, short version....... is to incrementally change the overall velocity, working up or down the scale to find the range where the rifle self-compensates for minor velocity changes in such a way that the slower bullets are launched higher or lobbed into the target to the same poi as the slower bullets. These points of convergence are often referred to as "nodes." They are yardage specific, in other words BULLET TRAJECTORIES CAN ONLY COINCIDE AT ONE YARDAGE.
I will call this "coarse tuning." (Others may feel free to call me an idiot)
These "nodes" may well be 1/2gr or more wide, I refuse to guess about "size" because I work with a lot of configurations. Nodes vary dramatically in size and effect. This width or "size of the node" is driven by factors which affect the "stiffness" of the barrel. (length, diameter, taper, profile, stock design, weight distribution, etc) and I've got systems that track up and down like a whale's tail. Others shoot nearly waterline allatime....
Now, "once you're "in the node" you can "fine tune" for hole shape, generally (but not exclusively) using seating depth and neck tension.
Actual poi changes at 100yds are the result of LARGE changes on velocity and in any case are not linear. In other words, going UP with the powder charge will often make poi go DOWN between nodes. Working up in .2gr increments (weighed) using an accurate rifle will result in a poi chart that, while rising in the main, will show waves like a tide chart.......I give Jim Borden and Doc Jackson credit for dubbing this effort "graphing the sine wave" but I may well be wrong.
I do know that IT WORKS
and that graphing loads on a horizontal line is telling.
I also know, I'M NO SHOOTER, I'm a backyard experimenter trying to learn to be a shooter. Thank you guys for sharing this info.
It's why I do what I do. And hope soon to do what you'se do
al