I used standard RCBS full length dies that I put more threads on the stem so it would be higher in the sizing die chamber. Or use a stem from a shorter cartridge's die. Should have mentioned that.The vast majority of my dies are older RCBS dies from the 1970s and 1980s and you can not raise the expander enough to duplicate the Forster design.
On a whim last year I decided to buy a Forster full length 30-06 die and was extremely impressed with the low neck runout readings right out of the die. With the existing RCBS expander assemblies I experimented raising the expanders and using rubber o-rings and did not get the low runout figures I did with the Forster expanders.
Therefore I do not share your results and wonder what type/make dies you are using that duplicate the Forster design.
I disagree. The expander's included for those who want to use them; nothing's mandated by Redding about turning necks first.The problem with your statement is you must turn the case necks to a uniform thickness and if you do not you are just pushing any neck defects to the inside of the case neck. And this is "WHY" bushing dies come with a expander for the people who do "NOT" neck turn.
Sierra's ballistic tech whose job was to test their stuff for accuracy told me to hone my FL dies out and toss the expander ball. He never prepped cases in any way sizing them with such dies and got 1/4 MOA accuracy with their best match bullets. As far as I know, Sierra now uses unprepped cases full length sized in Redding full bushing dies without expander balls for cases they're made for. I doubt anyone shoots their stuff at 200 yards as accurate as they do.
I tried neck turning to get them uniform but gave up as it didn't make any difference. A .0015" spread in wall thickness will make case mouths and throats a little bit egg shaped when they come out of a gelded die, both outside and inside measurements. But I seat round bullets in them; that makes the case mouth and throat round, too. Don't care if the outside surface has a small spread in its dimension to the bullet; that doesn't touch anything when the round's fired. Accuracy is excellent or better.