Safety first Muir, but why would you have an issue with starting at maximum recommended loads for the standard cartridge in an AI configuration? Otherwise there would be no advantage or point in the AI other than less trimming perhaps being needed. I accept that the shoulder angle will cause differences in powder burn etc and therefore pressure variations but it would be unlikely to show much of a benefit in MV using the same powder load.
This is the first AI rifle I have owned and interested to hear comment from those more experienced than me.
I got news for you: There is very little to be gained with an Ackley chambering. The case capacity difference between a 243 and a 243 IMP is 6.3%. The rule I have gone by, and it's an old one, is that velocity increases at 1/4 the increase in case capacity. This means that you will utilize a 1.5% increase in muzzle velocity...when all other variables are the same. These being barrel length and type of powder and pressures.
Look at it this way. If you increase the usable case capacity by 6% and leave in the same amount of powder, you would expect (generally) lesser velocity, would you not? For a given bullet and powder it takes a certain amount of pressure to push a bullet out of a barrel at a given speed. By increasing the case capacity you have just lowered the pressure (say max @ 60K psi) and most likely, the velocity. So. What to do? You increase the powder until you reach the same velocity. You have achieved this velocity by generating the same pressures, with that same powder, as you had with the standard load. So now you have added more powder and gotten the same velocity and pressure. Now you add more powder because the Ackley Improved cartridges are stronger, right?? Wrong. You're done: 60K max pressure. Why would you get even that 1.5% increase in MV? I'm not sure but most likely it is due to the increase in powder weight. The added weight of the powder itself must be accounted for in internal ballistics and this weight will change the burning rate of many powders. (Check Lee's book on compressed loads) In any event, the velocity difference would be about what you would expect in standard deviation on a chronograph and most likely be within the accepted norm of pressure readings. If AI shooters measured pressures they would get a hairy surprise. Here it is the custom for gunsmiths to suggest ridiculously long barrels for their Ackley chamberings. One friend of mine had a 30 " barrel installed. His velocity increase is about what you'd expect for such a long barrel and he said his powder charges are about what he used in his standard 243. One fellow on this board boasted velocities of his 243AI and a 105 A-Max... at pretty much exactly what the standard 243 obtained.
The classic "pressure" signs aren't always evident in fired cases, even when high pressures are at hand. Ackley chambers are notorious for hiding pressure do to their straight walls (and subsequent adhesion to the chamber walls during peak pressure) resulting in lack of back thrust... and the practice of neck sizing only. This doesn't mean that pressures aren't high. You just aren't seeing them.
In the Lyman #43 manual (IIRC) there is an article by Ackley saying that high intensity cartridges weren't worth "Improving". A 60K cartridge must remain a 60K cartridge. Ackley told me that the reason for improving a cartridge back in the 50s and 60s was so that people could use the slow burning, cheap, surplus powders in cases that were otherwise unsuited for them; like the 30-30 Winchester. He admitted that his experiments Improving many different cartridges were done because he had the means and wherewithal to do it...and he loved ballistic experimentation. He also admitted that for much of his work he ran blind with no access to a pressure gun and just the use of a Tipco Time Meter chronograph.
I am not saying that the rifle and chambering is bad. Many of my Ackley IMPs were tacdk driving screamers and I took them on that account. You will enjoy your rifle but remember, there is no free lunch with ballistics, and starting out on Max is silly. My opinion, of course.~Muir