ChrisC
To get back to your question of why breech seating is more accurate than fixed, I'll take a whack at explaining it. With fixed ammo, the expanding powder gas pushes the bullet from the cartridge case and through the throat before it engages the rifling. Even thought only slightly, the bullet base gets distorted before engaging the rifling, plus the distortion is never perfectly consistent from shot-to-shot. When breech seated, the bullet is already into the rifling ahead of the throat and perfectly concentric with the bore. Not having to make the jump through the throat, the not distorted breech seated bullet travels a more consistent (accurate) trajectory to the target.
Make sense?
BTW, Joe, that target is a nice clean 250. WTG

Maybe a word of warning is appropriate at this time. 

When you finally acquire your Hepburn and accomplish the very tough and illusive 250, Your status will sore. The lovely Schuetzen groupee chicks come out of the woodwork.  Best to leave the spouse home and keep a tight lip if you think you may score the big one. Be careful what you wish for is all I can say. 

In my case everything worked out for the best. By the time I scored my 250, was so old and decrepit the chicks took little notice. 

        Joe.