Basis for my opinions?
For starters, compared to most around here, I consider myself a newb......but getting up to speed as best I can. My goal is to load accurate ammunition for hunting purposes, and by accurate, my goal was 1 MOA for the 5 rifles I currently load for. Am close to that or better for most. So far, those that are good have been accomplished with nothing more than the right charge weight. Seating depth nothing more than close proximity to COAL used in load data.
Newbs can read any of several manuals, but past the basic process, most manuals will fall short of a defined load development process for accurate ammo. They take you up blind alleys, bounce around with stories, etc. but the load development process generally falls back to start low and work you way up to the max. Beyond that, they don't say how.
So next step for a newb is to go online where you find out optimal charge weight, etc. which explains more of the how of load development. Increments and such, but there are lots of variations on the variations. How big are your steps? In short, when you pull back the curtain to get a look at the wizardry going on, you discover there are all manner of variables that can affect results, and most of us cross thread so many of them, it's nearly impossible to sort out what causes what and why.
But as to optimal charge weight, what floats to the top as plausible theories.......plausible to me anyway.......is once you find a sweet spot......for whatever reason.......you hit an accuracy node. It shows up in chrony speed and it shows up on the target.
Take this target for example:
View attachment 1114476
That is a 15 shot load ladder, with target set at 200 yards. Powder loads increased in increments of 0.2 grains. As groups go, not too impressive, except for that group from loads 7, 8, 9 and 11. Random or an accuracy node? Chrony had those 4 or 5 shots all running about the same speed, so by all accounts, an accuracy node. How would you find it except for loading and shooting it? And that too, with random seating depth, no where near the lands. COAL same as found in load data, and within SAMMI specs. But as for testing, what is left now is to load some rounds in the middle of that range and see if it repeats.
So what happened to #10? It flew 8 inches to the left and missed the target entirely. WTH? Wasn't pulled that I know of. So why the flier? Impossible to say, but it certainly affected the group. Could it be the bullet was the twin to another I found in the box that was a full 0.10 longer measured at the ogive? I was working on a different caliber yesterday and 10 bullets varied a full 0.010"........measured at the ogive using bullet comparator. At best, there was a range of 0.003" in the middle as most probably length. According to some, 0.003" is a seating depth accuracy node, so if concept has value, how in the world can you improve on accuracy when the bullets you are working with won't allow it? Are the fliers on us or is there a limit based on what we have to work with? And that is the gross variables......not even into sorting brass for weight, bullets for weight and length, turning necks and all the other BR tricks.
Last road I intend to go up is to see if changes in seating depth affect accuracy. Some say yes, some (on here too) say no. I'm holding off until I get to test it myself.