I believe the change in seating force has to do with the setting of the structure after compression from the neck size reduction. Remember for dimensions to change the brass MUST flow. It molecules must move. If you read some of the peer reviewed research studies into the molecular structure and changes in brass from both annealing and working you will find observations of changes in the lattice structures of the brass. How the interlock and voids shift. I believe that is what you are seeing with the increased force to get the case neck to yield a given distance.

One thing to check is to precisely measure both the outside and inside diameter at points along the neck length. Test 1hr, 24hr, 72hr, 1 wk, 4 wks 8 wks. Also testing seating force on each set of cases. If there is a dimensional change that answers the question. If not the nicroscopic scans would need to be done looking at the molecular structure over the time periods.

One thing that could be established is how long after sizing does tension finally normalize and not change.

We know from hardness testing by AMP that actual hardness does not change over time but then that was not sized cases with intervals of time but only annealed cases. So maybe something to look at directly. Honestly this is the kind of thing a metallurgy doctorate student would likely enjoy researching.

I live in Denver and have time… anybody have access to one?

Another question or test I am curious of would be if annealing after sizing can mitigate the “set” the brass takes or wants to go back to. This is a great article and is timely to me as we have been discussing it amongst our rifle team.

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.

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.