My misprint on the mandrel diameter. It is .0005" smaller than bullet diameter. So, Technically, I have a 1/2 thou. neck tension (bullets are soft seated).
Al, I believe it has to be the bullet. I sort the bullets for bearing surface length and all are within .002" of each other. There is a ring on the bullets, in the ogive section from the seating stem. To my knowledge, I don't have a special seating stem, just the one from the factory. With the low neck tension, the ring on the bullets (visible on the moly coating) cannot be from neck seating pressure. The weight of the press handle seats the bullets.
In my 308, it could be a combination of low neck tension, and full cases of powder that the bullets are pushing on. I use a 6 inch drop tube on my funnel and try to get the powder to compact in the case as much as possible. The powder could be pushing the bullets back up with the low neck tension.
To get the same base of the case to the ogive of the bullet, I am working the seating stem on my die to hit the target OAL. It gets there and the target reflects good scores, but it is a lot of extra work to make them all OAL brothers.