From the vault: A reader-submitted question regarding the .30-06 Ackley Improved wildcat cartridge. As published in the October 2001 issue of Shooting Sports USA.
Q. What is a .30-06 Ackley Improved? How does it differ from the .30-06 Springfield, and can you fire the .30-06 Ackley Improved in a rifle chambered for .30-06 Springfield?
A. The .30-06 Ackley Improved is a wildcat cartridge formed by firing a .30-06 Springfield cartridge in a rifle that has been chambered or re-chambered for the "improved" version. Upon firing, the case "blows out" to conform to the new, enlarged chamber. The new dimension has a reduced body taper and increased shoulder angle compared to the original .30-06 Springfield case. By doing so, the volume is increased a small amount—less than five percent. You cannot get the fatter case of a .30-06 Ackley Improved into a standard .30-06 Springfield rifle chamber, but the reverse is obviously true. The velocity gain over the standard .30-06 Springfield is negligible.