I'm loading 900 rounds of 6.5-284 ammo for some customers shooting the Hornady ELD's and Berger 140's. I'm loading it on virgin Lapua 6.5-284 brass. Now, my brass prepping process for this ammo is to run the virgin brass case up over a standard 6.5mm expander ball in the sizing die. I don't size the brass at all, just run it over the expander ball to open up the case mouth some. When I seat an ELD bullet on virgin brass straight out of the box, the neck tension is so high that I can not seat a bullet without it causing a heavy ring around the bullet where the seating stem from the die makes contact with the bullet during seating. It leaves a heavy ring from the high compression of seating the bullet, which is no good! To solve this heavy neck tension issue, I run the virgin brass over the expander ball to stretch the necks to loosen up the neck tension. This brings me to my discovery. Last week I primed and prepped (necks expanded and VLD chamfer) this virgin 6.5-284 brass. But, I didn't have the bullets on hand to load last week, so the brass sat idle fully prepped.

Fast forward to today. I have the bullets now to load, and went to load the 6.5 brass from last week that had been fully prepped (necks expanded). When seating the bullets on the brass prepped last week, the case necks were back to the original tightness. When seating a bullet it would leave a ring on the bullet from the seating process. I ran some of this brass over the expander ball again (now twice), and it seated just fine with not much pressure and no heavy ring from the seating stem.

I'm amazed! I know we have talked about the cold metal welding of the cases changing up the loads. I also do think this tightening of the neck of the brass could also be an issue. The measured change from last week to the expansion today was .002" to .003" difference. I know that doesn't seem like a lot, but on virgin Lapua brass is A LOT! Lapua brass has been known to be very good brass allowing multiple reloads. But the virgin Lapua brass sometimes does need some case prep to get it just right.

This seems similar to something I ran into last week.

I load .300 Win Mag for a friend of mine. 100 rounds will last him about 2 years. He handed me a box of twice fired brass to reload. I cleaned, and annealed the case mouth, and made sure to get some heat getting down onto the shoulder well past the datum line. The brass looked just like freshly cleaned and annealed brass should look. The FL sizing die had been set, in the past, to bump the shoulder .002". Well, on this freshly annealed brass, the case head to shoulder measurement increased, instead of decreased. I had to run the die into the press farther and create a new setting to get the shoulders to bump. Judd and I decided it was spring-back from being annealed, and possibly the expander ball was pulling the brass longer on the way out of the die. It was strange, in my experience.

It may be your unintentionally sizing the neck when your running the expander ball through it. It could also be it wasn't properly annealed from Lapua. I use the same brass and the first thing I do with it out of the box is anneal it even though its been annealed from the factory I've discovered its not always done correctly and it can cause too much spring back when sizing.

No, not possible. I'm lowering the expander ball down inside the die and raising the body of the die up on purpose as to not allow the case neck to touch the neck portion of the sizing die.

I've had the same problem with Lapua I actually use 6.5x284 brass for a 6.5 and a .284, I would anneal the necks and try not get it as hot as you normally would past the neck shoulder junction then run the ex ball through it.

In my opinion the brass you have was not properly annealed and stress in the metal caused the neck to spring back

"A brass case head can be work-hardened to withstand the high pressures of cartridges, and allow for manipulation via extraction and ejection without tearing the metal. The neck and body portion of a brass case is easily annealed to make the case ductile enough to allow reforming so that it can be reloaded many times. "

I don't know if this helps but I had a Redding 223 rem seating die that left a ring on 77 gr TMK and 80 gr ELDM s. I changed to a VLD seating stem and that fixed it.

I have noticed, at least on some Lapua 223 brass, that neck wall thickness was more than expected. It was consistent, just a bit thick. Not sure how that would factor into this discussion, but thought it might be worth mentioning.

In my case, I would never have measured the neck wall thickness under normal circumstances. It just happened to cause a problem with reloads for a very tight chamber. I can envision that thicker neck walls would require more force to seat a bullet, and a thin skinned match bullet might get dented from the effort. Of course that does nothing to explain a case neck that would ‘shrink’ overnight. That really is odd.

It's the cold weather. We all know things shrink when it gets cold

The temps those days were in the 50's to 70's from the days the brass sat idle. Could it be because of cold, maybe, but I don't think so. And yes, the necks on Lapua brass are a little thicker. I have also noticed that.

When Lapua first started making 260 Rem brass, the Lapua brass would not chamber in some match chambers, like the GAP Precision rifles. Multiple gun smiths had to change reamers to allow for a larger neck diameter. It was an issue for a while on some 260 Rem rifles. The necks on that brass were very thick.

Not all brass is created equal and some like Lapua is normally some of the best brass you can buy. Bras used in cartridges has an elasticity that is one of the great things about it for that purpose to be able to expand to the chamber dimensions tight enough to form a very good gas seal while under extreme pressure then spring back a little to allow the case to slide freely from the chamber.

i wonder what would happen if you took a few pieces of that new brass and annealed it then sized and set aside if it would then spring back in to where it was or would the "softer" brass maintain its dimension.