Zip ...
Neck tension. When you preload, you lose neck tension.
Actually, that is not the case. It's the TIME between when you neck size and finally seat a bullet that seems to be the key controlling factor. Preloading is inconsequential.
Source:
http://www.6mmbr.com/BlogJan2006.html [2/3rds of the way down the page]
RELOADING TIP--Neck Tension vs. Time: We've learned that time (between neck-sizing operation and bullet seating) can have dramatic effects on neck tension. Controlling neck tension on your cases is a very, very important element of precision reloading. When neck tension is very uniform across all your brass, you'll see dramatic improvements in ES and SD, and your groups will shrink. Typically you'll also see fewer fliers. Right now, most reloaders attempt to control neck tension by using different sized neck bushings. This does, indeed, affect how hard the neck grips your bullets. However, James Phillips recently discovered that another critical factor is at work. He loaded two sets of 22 Dasher brass. Each had been sized with the SAME bushing, however the first group was sized two weeks before loading, whereas the second group was neck-sized just the day before. James noticed immediately that the bullet seating effort was not the same for both sets of cases--not even close. Using a K&M Arbor press equipped with the optional Bullet-Seating Force Gauge, James determined that over twice as much force was required to seat the bullets which had been neck-sized two weeks before. The dial read-out of seating force for the "older" cases was in the 60s, while the seating force for the recently-neck-sized cases was in the 20s. (These numbers correspond to pounds of force applied to the bullet).
Conclusion? In the two weeks that had elapsed since neck-sizing, the necks continued to spring back (get tighter) and stiffen. Lesson learned: for match rounds, size ALL your cases at the same time. If you want to reduce neck tension, load immediately after sizing.
Here's more:
Should You Bench Load ???
The majority of top 6PPC competitors load ammo on match day, playing with load weights to suit the charge to temperature and humidity. While this allows you to get peak velocity on a cool day, and avoid over-charges in very hot conditions,
"bench loading is a huge distraction", and isn't essential.
Hall of Famer Jim Borden: "I have my ammunition already loaded for [major matches]. It's working, both rifles are Agging in the 'low Ones', with a load of about 29.0gr N133, running about 3300 fps."
"Loading to conditions on match day can kill you. All you can do is go by memory and guess-timates. If your rifle has good vibration control, you'll typically have an accuracy window of 100-125 fps."
"I set preloads for velocity in the middle of the accuracy window which lets me shoot in any conditions. Studies by Jack Jackson convinced me we could let ammo sit for as long as a year and accuracy will hold."