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.
Similar Posts:
- TECH TIP–Neck Tension vs. Time
- Neck-Expander Mandrels for More Uniform Neck Tension
- Expander Mandrels — Not Just for Neck-Turning
- The Complexities of Neck Tension — Why Bushing Size is Only One Factor to Consider
- Expander Tool for New Brass
Tags: Brass, Neck Tension, Reloading, Time
It would be interesting to shoot the ammunition,over a chronograph, under good conditions, over wind flags, first with similar seating force shot together, and then mixed in the same group, comparing the results with historical data from the same rifle and load.
It would also be interesting to know if there is a difference in tension between rounds seated at the same time but tested right after loading and at some later interval. Perhaps a week, two weeks, a month?
Just when I think I have reloading figured out! What’s next take them to dinner and a movie?
And which batch had the lower SD?
To mitigate this phenomenon, I run a Sinclair carbide turning mandrel through each neck -just prior to seating.
This produced more consistent seating forces as measured using ‘LoadForce’. It also drives some thickness variance outward which results in lower runout off the ogives.
But I have not done any official time testing of aged loaded ammo over a chronograph.
Wonder what effect storage temperature such as freezing wouild do?
I have always tried to have freshly prepared match ammo with my necks sized as close to the match time as possible. Having said that, I sold a 300 WSM Heavy gun that I last shot in 2006, along with the remainder of the ammo not fired — 187 BIBs over H4350. The new owner shot that ammo in a 1000 match in December. He shot a 99 score on one relay and and 5.2″ group on another. The ammo had been loaded in March, 2006 — not what I would want to shoot, but what can I say.
Jim Hardy
I am a retired A/C & R contractor and one of the things that I noticed over the years is that a rool of dead soft copper tubbing when left in the cardboard box would stay soft, BUT take that roll out and hange it on the wall and within 2 weeks it would be harder than harder than hell. go figure.
I am not a metallurgist so I can’t give a fully rigorous explanation. However, its generally known that when brass is sized it tries to return to the shape it was before sizing. Perhaps we don’t fully understand how long it takes for that tendency to continue. So, if one sizes a number of cases and does not load them all the unloaded cases can continue to change more than the loaded cases. It would be interesting to know if there was a significant difference in neck tension between when the cases were initially loaded and sometime after.
Cold working most metals results in crystalline lattice deformation, reduction in ductility and increased tensile and yield strength. The effects are caused by increased dislocation density within the lattice structure, behaving as if the material were denser. Annealing above the recrystallization temperature allows the material to assume a natural structure, achieving maximum ductility and minimum strength. When 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.
It is for this reason that I anneal cases every three firings and size only to the extent necessary.
So might there be an advantage to anneal necks of sized brass (that’s been stored for some indeterminate time) just prior to priming & loading?
Yes — at least if you accept the description of the testing results in the article: brass’s crystalline matrix continues to change over time. Loaded rounds may experience less of a neck tension change, or perhaps a different kind of change when compared to sized but unloaded cases, as time passes.
If you size and load ammo in one session but shoot this ammo over a period of weeks, will the neck tension change over time?
I’m among those who think annealing is extremely important… especially if you can’t “force sort” the brass because you don’t have a tool like the K & M Arbor with Force Measurement.
My suspicion is that you would find far less difference between brass worked any number of days and weeks before and same day brass if the necks were properly annealed.
Regardless how much difference remains, say you had 2 week old brass that all took say 60-65 lbs to seat. Then you had same day brass that has 20-25 lbs. Which of these two groups is going to have more force SD the next morning when you go to shoot? I can’t answer that because I’ve not tested for it. I think I would prefer the older brass. The reason to me is that it’s “settled” where the same day brass may have very inconsistent overnight changes.
I think I would feel good if I used old brass and “force sorted” into say 60-62 lbs and 63-65 lbs than I would 20-22, 23-25 lbs on the same day brass.
Just my 2 cents.
I’ve seen several spreadsheets where people have used the K & M with force measurement. Frankly, I was not impressed.
I’ve been working on a tool, unperfected at this time that does it differently.
When I size my .308 cases, the ID is usually .305 after neck sizing with my Lee sizer. So I bought some pin gauges in varying thicknesses from .0300 to .310 in .001 increments.
My (theory at this point) is that you’re better oof inserting a .306 pin gauge into a .305 neck opening and measuring the force required to extract it rather than the force required to insert when you’re seating the bullet.
First, I think it would be a more accurate means of determing how much force it will take for the burning powder to dislogdge the bullet. Both are going in the same direction.
Second it’s much more efficient. How many rounds do you have to make up with the K & M in order to have 100 for a match? You could be left with 10′s, 20′s or even 100′s with varying neck tensions.
With the pin gauge extraction method, you sort the case by extraction force as you’re doing the measurements. When you get to 100 of one measurement, you’ve got what you need for the match. Say of those left over, you had 30, 40, 25 and 30 of other measurements. Next time you need a hundred for a match, you start doing the extraction force method until either the 30, 40, 25 or 30 pile reaches 100.
The only problem I have is not having the tools and/or materials to make the design I have in my mind.
I know intuitively that extraction force is better than insertion force, I just can’t get it set up.
Just FYI, if you put extract the .306 pin gauge from the .305 ID neck, you’ll still have plenty of tension for the .038 bullet.