[Larryjk]

Mike Moss, I don't understand your response to Westerner2.
(1) Your first paragraph is a IMHO. This will always be disputed.
(2) Irrelevant. I met P.O.Ackley once and that is irrelevant in this case.
(3) Ackley Improved chambers are 0.004 shorter in headspace than the parent chamber so the case web doesn't get stretched during the fire-forming process. This in no way "limits the chamber flexibility". If you think so, please explain your reasoning.
All chambers are specific to the chambered cartridge and minimal headspace is always what is desired; flexibility be damned.
(4) I basically agree with what you say in this paragraph and the next. However, I might add that there is more data all the time on the .280 AI. Unfortunately, most of it is gathered without the pressure being considered. Of course, anyone who takes an "excursion" beyond the strict limits of the data in a loading manual is doing the same thing. Remember, each barrel is a separate entity because they are not identical. An average of five different barrels with the same ammunition only gives you the average of five barrels. The next five barrels tested may very well give a different average.