You are entitled to your opinion, but not to make up facts.
Yes, theoretically if I take a large cylinder and compress it to form a narrower cylinder the wall thickness has to increase. So I will give you that, but that is only in theory. In the real world things are more complicated.
Most virgin brass manufactured necks have as much as .003 differences in neck walls and will easily fit in a SAAMI chamber with simple neck sizing and generally will have excess head space. It isn't unusual for Lapua or Norma brass to have .005 or more of head space. Sometimes its even more and I have to jam the bullets to keep the brass from being pushed forward by the firing pin therefore failing to fire. That's why most competitors don't take virgin brass to matches, it isn't fire formed to the chamber. After the brass has been fire formed most of us precision shooters only push back the shoulder a couple of thousands. After pushing the shoulder back .002 it is still forward or where it was when in its virgin state.
So your claim that pushing the shoulder back creates donuts is false.
I uniform (I think the term "turn" hides the real purpose) my
virgin brass, and after I've fired the first time I uniform the necks again. Guess what happens? More brass is removed. Where did it come from? Pressure, 60,000+ psi, and heat drove it forward. Why? Very simple, that's where the pressure release is.
Brass flows forward, because ignition starts at the case head with primer blowing everything forward because of
heat and pressure.
Yes, Ackley Improved cartridges reduce brass flow, that is the main benefit of AI, because the 40 degree shoulder provides more support to forward pressure. However, it doesn't eliminate it. I know I have 260AI.
In theory one should be able to ream a donut out of the case, in reality that is not practical because getting the geometric dimensions just right (wall thickness, and neck size) is beyond the ability/finances of most reloaders. So, the best most of us can do is push the donut to the outside and remove it with a neck turner.
The higher the pressure you run, the more likely you are to have a donut. If you can seat the bullet forward of the donut, that's the way to go. But, remember heat and pressure drive brass forward and with enough firings the brass will move to your bearing surface and affect neck tension, bullet alignment and bullet release.
Kindest regards,
Joe
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Or, you could just use no-turn brass and get your freebore such that the bullet is never close to the donut. That would be the way to solve this problem with a lot less work! We use about .250 FB on our 284s and it works like a charm. Have converted a bunch of people over to the dark side (no-turn) and they have been very pleased (and surprised) with the results.
