You cannot fireform an improved case in a standard chamber.Like BruceP stated,you need a rifle chambered correctly in the improved version to fireform.When done properly,an improved chamber is .004 shorter than the standared chambering thus insuring the "crushed fit".When converting to improved it's imperitive that the barrel be set back to achieve this.This is the way Mr. Ackley design it.When I rechamber to an Ackley,I set the barrel back a half a turn so as to put the sight screw holes,lettering on bottom and also insuring a completely new chamber.