This has been a few years back but is precisely to the point, I think. I was shooting my 30 Major, formed from excellent 6.5 Grendel brass. FWIW, it uses small primers and small flash holes and necks were turned for a .330 chamber neck. IME, that brass is equivalent to 220 Russian brass formed to 6 PPC.
I was struggling during a particular match and had been for at least a couple of matches prior. It was progressively becoming clearer that the gun was double grouping....3&2's etc, but clearly double grouping. I was scratching my head and of course blaming conditions and such for a while. I had thrown everything at it but the kitchen sink and new brass. Told the guy next to me, who is an excellent and very experienced shooter what I had been fighting. After listing and ruling out several possible causes between us, he asked how old my brass is. I kinda doubted it but I had exhausted just about all other causes. It had 50 firings on it and not at all pussycat loads. Back then, annealing was not nearly as commonplace as now but I had annealed it once at about its midlife..around 25 firings.
Bottom line, I went home and made up all new brass and it was like a new gun. It solved it, no doubt. It's still the only time I've gone that far with brass and it was a couple or so, too many in my case. There are variables, of course...like pressure, the annealing and how much I was working the brass with each firing/sizing. I typically run pretty heavy neck tension in the 30's and I tend to prefer a tad more aggressive sizing than some. All factors have that some amount of value in all of this, so definitely, ymmv and I don't think you can possibly put a definitive number on brass life, aside from a swag at it. But in this example, it was absolutely the cause of my issues. I know well that there are examples of far less as well as more firings on brass but unless all things are identical, it's apples to oranges.
It was definitely a case for head scratching as I had never seen that exact scenario play out. With my 6's, I typically lose the primer pockets well before any of that. Even if I didn't, I will never push brass that far again because it was just to hard to diagnose until it became clear and I wasted quite a bit of time and money before I got it shooting right while it was the brass all along. Easy enough in hindsight, but do not assume that brass will last forever. It won't. It might seem like it with moderate loads and such but it definitely has a point where it's not worth it to try to squeeze a few more shots out of it.
These times we are in at the moment are different and hopefully not the new normal but keep some new brass on hand if possible and save yourself a major PITA.
