You can buy 6mm neck sizers. Hornady used to sell them. I second the chamber cast idea, unless you're in contact with someone who has fairly intimate knowledge of that rifle. A P.O. Ackley stamp on the barrel does not mean it has the Ackley chamber he was famous for. He also made standard designs, various different shoulder angles, same with neck lengths. Lots to figure out. Pretty cool rifle though, and a fine deer cartridge in that gun, assuming enough twist. - dan