READ THIS from 6mmBR.com:
February 13th, 2010
Neck Tension, Bullet Seating, and the TIME FACTOR
This may surprise you. We’ve learned that the time interval between neck-sizing operation and bullet seating can have dramatic effects on neck tension (as measured by the force required to seat bullets). 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 firmly the neck grips your bullets. But time of loading is another key variable.
James Phillips discovered that time is a critical factor in neck tension. James 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 much more force was required to seat bullets in the cases 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 loosely correspond to the amount of force required to seat the bullet). Conclusion? In the two weeks that had elapsed since neck-sizing, the necks continued to get tighter and stiffen.
When we first posted this information, it spawned some debate. Many people said they have observed the same thing, but the question is why? Something seems to happen over time that makes the necks less “springy”. Our theory is that, over time, the necks (as sized) are taking a “set” and seem to lose elasticity or the ability to stretch. When they are freshly sized, the neck material seems to be more ductile and expands more readily as the bullet is seated.
In a comment to this post, Steve Blair offered this explanation of how case necks can change over time: “When [metal] material is cold worked, the lattice stresses induced may not be uniform and immediately realized. The grain structure can continue to change for some time, becoming harder and less ductile as the lattice deforms further. Seating a bullet in a case neck provides ongoing radial stress to which the metal will respond over time.”
Concerning the seating force numbers (20 vs. 60) — keep in mind that the K&M simply has a dial read-out activated by a Belleville washer stack with a link rod. This isn’t an ultra-precise measure of force. But you CAN feel the difference between a 20 dial position and a 60. If you use the K&M you’d see what I mean -– the needle tends to swing back and forth as the bullet is seating. What you want to watch for is the max reading and “spikes” in the seating force. I think what is going on is the resistance to seating goes up as the brass becomes less elastic over time.
Lesson learned: For match rounds, size ALL your cases at the same time. If you want to reduce neck tension, load immediately after sizing.
Whether or not you accept the notion that case-neck bullet seating resistance rises with time (you’ll need to do your own experiments), it makes sense to size all your match cases at the same time, and then seat all the bullets you need for a match at the same time. If, for example, you need 200 rounds for an upcoming match, you don’t want to size all 200 cases and seat 100 bullets the same day, and then load the remaining 100 rounds three weeks later. Almost certainly you’ll find some difference in neck tension. That variance in neck tension may show up on the target.
This brings up another point — to minimize velocity variances from round to round, it makes sense to shoot the ammo you load in the same order it was loaded (or exact inverse order). That way, if you have some scale drift over time, causing small changes in powder charges, the shot-to-shot variation is reduced.
