Just saw this post. Much of what I am writing refers to info Out of the recent Fred Zeglin book on PO Ackley.
PO did not originally conceive of improving cartridges to get more power. He did so because client were bringing him lever guns experiencing extraction problems. Straightening the case reduced back trust and significantly reduced case growth at normal pressures.
The AI design assumed the same level of pressure would happen. But as most of us know, if you reduce the back thrust against the bolt, one of the key, normal sighs of pressure has been redefined. Add to that the increased case capacity about 10 % more powder would be needed to achieve the same pressure as the parent cartridge, but the pressure signs would be absent. Human nature, would want to cram more powder in until the pressure signs return. So when the back thrust returned to the same indicators as the parent, it would actually be at a higher pressure.
Again quoting PO from the book, PO did not care what the pressure was, he only cared about the back thrust, hence the actual stresses on the action. That is why AI cartridges are loaded as they are, not by design, but by practice.
PO arrived at all of this by actual testing, not theory or opinion. Great book about a great gunsmith, who actually field tested old wives tales for the truth.
Dan
