A handful of things here:
*It goes without saying, but item 0 here is to not mix headstamps. Precision competition ammo will not be made in mixed headstamp lots of brass.
**Item zero point b would then be - don’t overthink it. The differences we’re talking about here are all on the razor edge. As an example - I don’t worry much about fireforming my brass for PRS competition. I’ll start with a batch of 400-1000 pieces each season, do my barrel break in and load development with ~150-200 pieces, and usually start shooting matches. For some batches, I’ll practice enough before the first match of the year to get all of my brass fireformed ONE firing, but usually not.
1) Quite often, new brass corresponds to new barrels, and new barrels will need 100-200 rounds to break in, so doing load development too early just means you have to do it again. The shift between virgin bore and seasoned bore is usually small, but sometimes significant enough to change loads (node moves).
2) Yes, brass will spring back more and not quite represent the actual chamber dimensions when it is new/young. When sending out for custom dies, it’s best to send 3x fired brass because they best represent the chamber dimensionally. (This has absolutely nothing to do with overpressure benchrest loads, just a metallurgical feature of brass and steel). The shift between virgin brass and fully fireformed brass is usually negligible, but sometimes significant enough to change loads (edges of nodes shift). This tends to be much less influential than barrel break in.
3) The above truths leave us in a hen and egg paradox - we have to shoot SOMETHING to get our barrels broken in and get our brass fireformed. Personally, I USED to do a quick 10 round fouling, cleaning between each, 10 more fouling, then a 30 round ladder. This would point to a node, then I’d load ~100 rounds of that load to break in my barrel. I reconfirm my nodes before almost every match anyway with 34 rounds fired, so I track my nodes throughout the life of a barrel and across all firings of my brass’s life. Within component panics like this, I accept a little more risk and will shoot those 100 break in rounds during a low level competition.
4) Starting with non-virgin brass will mean your brass is work hardened more than virgin brass, so it may take 3-4 firings to fully fireform rather than 2-3. Whenever I change barrels/chambers with brass, I check head diameter to see if I need to small base size (original chamber larger than new chamber, or under-massed gas gun), and I’ll typically anneal to bring the neck and shoulder back in check.
5) I never mix lots or firing counts of brass if I can avoid it. I always anneal if I’m doing so, and only mix known quality brass, and I still confirm case weight and internal capacity. I’ll also check seating pressure consistency and POI and MV consistency before mixing - loading the same load (after capacity sorting and Annealing) into each lot and testing replicate ladders and replicate groups. As an example:
@South Prairie Jim just shared in his Long Range Load Development thread that he identified a difference between two lots of brass in his Benchrest ammo which was causing variability in his muzzle velocities, and inducing extra vertical dispersion downrange.
6) Annealing is its own topic here, but I’ve mentioned it several times. Different firing counts will yield different levels of work hardening, which will yield different seating pressures, different neck tensions, and different grip pressure - meaning different primary ignition. Mixed lots will be more prone to stray with multiple firings, and differing firing counts will obviously introduce variability - so annealing is pertinent for this kind of plan to reuse and potentially mix lots of brass.
7) Node confirmations - as mentioned, I check my nodes before matches, 10 rounds to foul and 24 rounds for the test (triplicate of 8 charge weights). If my brass or barrel changes, I know it before I load for the match. Usually, nodes don’t shift much, and usually velocity doesn’t shift much, but if and when they do, I can see it before it bites me in the ass at a match.
So in your shoes, I’d be sorting by headstamp, sorting by internal capacity, Annealing, sizing to headspace in my rifle, comparing nodes & POI, and then deciding if mixing in the donor brass is worth doing.
