You have better equipment than I do (AMP). I can't say that I have found the same sweet spot that you have, but in a several year quest, I learned a lot as I worked to improve my TIR. I will add what I have learned:
Expander, if I am going to use and expander, I make sure I am not making it increase the case neck diameter more than 0.001". If it has to expand the neck 0.002" or more, it WILL pull the case neck off center. And I run my expander high so it passes through the case neck just as the neck leaves the neck portion of the sizing die. And I use carbide expanders and lube the inside of the case neck with my case lube. Doesn't matter, it still pulls the case neck off center.
Shoulder bump, I use Bootleg case lube (liquid lanolin and alcohol) because I have found it to be an excellent lube and it doesn't cause dents since it is almost impossible to apply that heavily. Uniform and consistent application of case lube is my key to a consistent shoulder bump. I also polish the interior of my sizing dies. I don't know if that helps with the shoulder bump consistency, but it definitely reduces the sizing effort. I find the case lube easy to remove also.
At the start of this quest, I tried using an O ring under the die lock ring. My goal was to check and see if floating the die in my Redding Big Boss press, helped with neck straightness. I didn't ever find that I could quantify improvement due to the O ring on straightness, but it did provide one other benefit. I can now make the minute adjustments to the position of the die to get exactly the shoulder bump I want. I really wasn't happy with the various die lock rings I have used (the split ring design need to be tightened not in contact with top of the press, or they are difficult to loosen, the set screw type can distort the threads and make the lock ring difficult to make small adjustments). Using the O ring friction of the top of the press and the lock ring, I can make adjustments to the die position as small as 0.0005" (which is difficult to measure consistently with my equipment). I don't even tighten the set screw at all. I make a reference mark on the die to know where I want to start and adjust from there - if necessary. With your Coax and the Coax lock ring, you avoid most of these issues. I have a Coax but I only use it for bullet seating, not for sizing.
I totally agree, if you aren't measuring, you don't know how the changes you are making are impacting things. I started with a Hornady concentricity gauge (now sits on top a cabinet). Next I got the Sinclair tool and it was much better. But it also now sits on top the same cabinet. After I got the 21'st Century concentricity tool, the others became obsolete.
Thanks for sharing about the sweet spot. I will spend some energy trying to see if I can find the same thing.
Thanks for the great post. I too, have been on a quest to minimize runout. I've been at It for about 2 years and have collected a lot of data and learned a lot on the way.
I went through my entire reloading process measuring runout through the process.
I stolderarted with a Redding Boss press. When using it, I found these things made the biggest impact on reducing neck runout.
1. Start with good brass. Lapua, or Peterson
2. O-ring on the ram to allow shell holder float.
3. I placed a flat stainless steel machined washer on top of the shell holder, ran the ram up to contact the bottom of the die, putting pressure on the die to take the slop out of the press/die thread contact. I held the die in place and tightened the lock ring.
This process is a pain if you have to adjust the die up or down, but I got pretty good at it.
4. Switching from f/l bushing sizing dies to f/l non-bushing sizing dies. I use a ball that is the .001 less than bullet diameter. Forster and Whidden dies produced the straightest cases. This is when I found the "sweet spot".
This got me to bullet seating as the culprit causing runout. I tried Redding competition seaters, Whidden Microtop seaters, and Forster non-Microtop benchrest seaters. Statistically, they were almost dead even. I switched to the co-ax press a couple of months ago. The sweet spot still works in the co-ax press.
The Forster seater produces the straightest seated bullets. The Whidden seater is a close second as long as I use a seating stem custom ground for each bullet I shoot.
After all this, I was able to conclude that for my 2 custom rifles, 6.5x47 Lapua and .223, runout of .003 or less will shoot into the group. My 6mmBR is a Cooper Varminter, runout of .005 or less will shoot into the group. I try for runout as low as possible, but realistically, I'm happy with 95% of loaded rounds having less than .002 runout. That's where I'm at now. 2 years and lots of water (and other stuff) under the bridge.
BTW:
Here's some stuff I tried that did not help. Some made runout worse.
1. Lee collet dies.
2. Vickerman in line seater. Could not get a bullet to seat straight at all.
3. Graphite lube in the case necks for seating. Basically had no effect, other than being messy.
4. Redding and Whidden bushing type sizing dies.
PopCharlie