Here's the method that Ken Howell suggests and that I've used with good success.
Dr Ken Howell
All but one of the methods that I've used produced off-center necks. Fortunately, the only one method that I can recommend works perfectly � fire-forming with inert filler. I've used a hydraulic case-former, a tapered expander button, a series of expander buttons (.308, 8mm, .338, .358), etc, and found 'em all disappointing. Fire-forming was a most-welcome discovery.
� Prime with pistol primers.
� Lube cases with good case lube.
� Load a test case with 10 to 12 grains of Bullseye or other fast pistol powder (generally, I go light with a .25-06 case and use a bit more with a .30-06 case).
� Wad � sheet of toilet tissue into a loose ball and tamp it down lightly atop the powder charge.
� Fill case to the mouth with inert filler (Cream of Wheat, Instant Ralston, corn meal, etc) � but not flour.
� Close mouth with smear of Vaseline, cast-bullet lube, paraffin wax, or a disc of paper from a paper punch.
� Insert case in chamber with muzzle up.
� Fire.
� Inspect test case. If shoulder hasn't formed full and sharp, increase test load slightly and try again.
