.338 Edge -Neck turning and accuracy

[Topshot]

I have been doing some load developement work on my new .338 Edge and thought some of the experienced shooters here might give me some ideas.
I have necked up .300 RUM cases to .338 then neck turned the cases before firing down by .0015" to even out neck wall thickness. I also loaded up some cases without any neck turning to get some comparision.
The rifle has a .370" chamber neck diameter and these fired cases come out with a neck diameter of .369".

Now the neck turned cases shoot alright, but the cases that are not neck turned shoot better! This seems a bit strange to me but it is what it is and quite repeatable. Anyone give me an idea why this might be?

The difference is so much I have considered not turning case necks at all. Is this a good idea? Maybe I should have neck turned the cases after the first firing?

I also have a .364 bushing in my D.E supplied custom Redding full length sizing die. I was thinking about getting another bushing either .365" or .366" to reduce neck tension. How much undersize do bench rest shooters etc, usually size their cases necks compared to chamber neck diameter?

The pressure needed to seat the bullets with my loading press is a bit more with the non turned cases compared to the turned cases which is expected. How much seating neck tension is desirable for best accuracy? More or less?

Thanks for your advice.

 

[dirtball]

If you used the same bushing for both cases, then the ones you turned will have LESS neck tension than the ones you did not turn. If the rifle shoots better with the unturned cases then I would probably try the next smaller size bushing before I went to larger bushings.
The bench rest guys I know experiment just like you are doing and use what works.
Most use light tension, but I know one that has to put .006 neck tension on one of his rifles because that is what that rifle likes.
Dave