The 30-06 Ackley Improved is made by firing the standard 30-06 in the Improved chamber. Headspace is the same, but the Improved case has a more abrupt shoulder, less body taper and a larger shoulder diameter. The most popular version was developed by P.O. Ackley in 1944, but there are other versions as experiments og back to 1940 or even earlier. This has always been a controversial cartridge with its detractors claiming it was not as good as the standard '06, and it defenders claiming it was better than the 300 H&H; Magnum. Actual chronograph tests have proven it to be definitely superior to the standard 30-06 cartridge with slow burning powders, but not with medium to fast burning powders. The 30-06 Ackley Improved is one of the most popular and widely used of the Improved breed. With the proper powder, it will add a little over 100 fps muzzle velocity to any bullet weight, as opposed to the standard factory loaded cartridge. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.