The neck on an Ackley Improved chamber is typically about .004 shorter than a factory cartridge. Just enough to get a crush fit for positive case forming. What is changed is the taper to the body of the case which becomes fatter at the shoulder and the angle of the shoulder is generally increaed to 40 degrees. Other improved versions exist whixh use different angles and some do blow out a portion of the neck. Ackley's chambers were designed to allow use of factory cartridges if need be and to allow simple fire forming.
