Different throats have different profiles which cause different amounts of resistance which gives different powders different time to achieve optimal pressure for the barrel.
Try sealing the barrel in with some asphalt tree sealer and doing another workup with both powders. Years ago people found out that military 45 ACP ball ammo was more accurate because it allowed better ignition prior to the bullet entering the bore.
Machine guns are awesome until you have to carry one.
Last edited by Jimro; December 30, 2012 at 12:28 AM.