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James--if you have factory guns shooting that good with factory ammo just by changing seating depth....and doing 1/4-1/2 MOA something tells me you've done a few tricks to the guns....it isn't just neck turned ammo.
Would you mind telling us what "factory" guns you're shooting and what you've done to them??
Have you ever run your own experiment on ammo with near zero runnout vrs ammo with say .004" runnout in these guns??
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The factory ammo I tune is obviously not neck turned.......
I will not load for somebody due to liability issues.... But, I will take their factory loaded ammo that they supply me and adjust the seating depth to the point where the gun likes it. Ruger #1s and Remington 700s without anything else done to them whatsoever and if you can't get them between 1/4" and 1/2" at 100 yards, then you are not done with it yet. I tuned my personal factory Ruger #1 to where it would shoot in the zeros for three shots at 100 and it had a sporter weight barrel. It is really not difficult to acheive these results if you feed the beasts what they prefer AND pay attention to the most intricate of details. I was going to play with the triggers, but decided that it would be a waste of time with stock factory guns.
If you knew the amount of freebore differences between manufacturers with the same caliber, you would wonder how some rifles even shoot 4" groups at 100 yards with over the counter ammo.
Finally, yes, I have run my own experiment and thus the reason I am adimant about the expander ball. The closer to zero runout, the smaller the groups were and vice versa. I initially had the same process that you use and finally decided to listen to some who have more experience than I and I was glad they took the time to try and help me. Since then, not one expander ball has ever touched the inside neck of any of my cases.......... That is except for revolver and semi-auto pistol cartridges.
The small amount of time it took me to turn the necks on my cartridges to eliminate any concentricity error was worth it to me because I attempt to make them the most accurate that is possible. After that, any shooting error or shot disperions is solely pilot error.
James