Now don't quote me on this and I expect I will be blown right out of the water but, I do believe I read somewhere that in days of yore old man Ackley came up with the improved cartridge to get a little more powder in and therefore a faster more efficient projectile. However the improvement in modern day powders has all but negated any improvement, and to all intents and purposes the AI round does not produce the advantage over the parent round that it once did.
swede got a ackley 243 i use for foxing used for the last two months . previously six years as a standard still getting used to it at moment shot a vixen last week at 320 with a 2 inch hold over would have to held 6 to 8 inches with standard . ackley very flat very fast .
I once used a 243 Improved in a Remy 700- could n't tell the difference between it and a standard 243 when firing, but the sharp neck on the case meant feeding from the magazine was terrible.
As Jayb says modern propellants mean you can get the extra velocity out of the standard case - indeed have a look a the sorts f velocitys Hornady light magnums etc are getting - they are getting into 300 magnum terratories.
I shoot one and wouldn't but for the fact it is chambered in a desirable rifle that was sold to me as a standard 30-06. It was found to be a 30-06 IMP after firing. In using this rifle I have found little or no change in velocity without a corresponding addition to pressure. Ackley never intended already efficient or "over bore" cartridges to be "improved". As the 30-06 is about the perfect case capacity for it's bore diameter, it gains little.~Muir