As pointed out above, If you headspace an Ackley chamber off a regular go gauge, you now have a chamber with at least 4 thousands headspace, likely more.........that won't give you a crush fit to hold the case neck against the chamber to allow the shoulder to blow forward to create the new Ackley dimensioned shoulder. That results in a situation where the brass grows in length and may lead to case separation problems with the brass down the road. The end result of a proper fireforming process is to create the Ackley dimension with a very slightly decrease in brass length (as the shoulder blows forward it pulls the neck towards the brass slightly).