JR: I knew Ackley to the extent that we corresponded regularly for a little over a year. It began with me disagreeing with him in a gunsmithing column in "Shooting Times" (US) magazine about forming .256 Winchester Magnum from 357 Magnum cases. He ended up writing to me personally and so the dialogue began. I later attended classes at Trinidad State College where he taught and did much of his research, tho he had passed away by then.
Within our correspondence, we discussed ballistics and eventually the Ackley shoulder. He believed that it promoted a more efficient burn but that is something that, with the exception of certain combinations of case shape and powder, has been largely disproved at last look. One of the reasons to look into the Improved shoulder was the availability of cheapo, slow burning military surplus powders. 4831 was sold from barrels in hardware stores at one time, and it was cheap. Guys were loading it in .22 Hornet and 222 Remington. There were many cartridges in which a case full of 4831 just didn't generate any pressures and low velocity. It was these cases that Ackley's shoulder benefited the most. A person with a bolt action 30-30 of good quality had two problems: the long tapered case that tended to blow the column of slow to medium slow powder down tube before useable pressures could be obtained (one of those special combinations I spoke of) and the small case capacity for these powders. The Ackley chambering solved these, or improved these to a great extent. Thirty-aught-six shooters could stoke their 06's a little more, even.
What Ackley thought a waste of time for practical applications was improving a high-intensity cartridge. A factory round developing maximum allowable pressures (and they are built to do so, trust me) can hardly be improved on by adding case capacity. If one sticks to the approved pressure limits, then you pour more powder into the Ackley chamber until you reach that pressure again. If the bullet and barrel length are the same, you should see little difference in velocity and of course, we already mentioned that the pressure was the same. Unless you have existing barrels rechambered with an Ackley chamber, it's impossible to realize unless you have a pressure test gun. Here in the US gunsmiths who specialize in rebarreling gently push customers towards longer barrels. That will give you velocity increases od substance. A buddy of mine just got a 243 AI with a 30 inch barrel. You bet! More speed!!
I believe that Ackley did much of his experimentation because he could. He had marvelous equipment and the freedom to use it. One of his students, Deward Finley of Raton, NM attested to that. I apprenticed with Deward who was a fine barrel man and an admirer of Ackley. One thing Deward told me was that, tho Ackley's books were popular, the real "skinny" on Ackley's chambers was written by Ackley for the Lyman Gunsite Company in their #43 (IIRC) reloading manual where he talks about the kind of cartridges that should be "Improved" and those which should be left alone. My coments above about hi-intensity cartridges mirror what he wrote there.
That anyone's "Improved" chambered rifle is a fine gun and a serious shooter, I have no doubts. My 220 AI Swift was a screaming tack driver and I have often wished for it back. That you get better case life is believable, too. Case life on my Swift was far greater than in the standard chambering. I don't know if I got more speed, but the brass lasted a bit longer. In my last IMP gun remaining, a 30-06 IMP, I get no increase in speed worth the conversion and dies.
I have nothing against Ackley chambers really, Just don't tell me that a high intensity cartridge, now improved, is better than the parent round unless you bring me pressure and velocity data to prove it. Tel me it's kicka$$ round and that it shoots great and that you love using it,and I'll give you, and the cartridge, two thumbs up.~Muir