Good point
@joe16
So, a little background on my question...
I've always been a once-fired until it's wore out brass reloader, whether commercial or LC brass. With that experience, I have noticed minor changes as the brass ages through the number of firings and trimmings. Not enough to make any truly significant difference in a hunting scenario, but when I was shooting long range (1K - 1400 yard) target using LC converted to 6.5CM, I would limit the cases to 3 loadings and then they went on to other rifles until they got wore out.
It wasn't until recently that I purchased some Peterson 300 WM Long that I started to ask the question.
Originally, when I bought 100 pieces of Peterson, I measured a handful for overall length to see if I needed to trim them all to a consistent length, nope, no need, so I just loaded up and went searching for pressure, found my load and ran with it.
I now have 50 cases that are once-fired, and FL resized to the same shoulder length as factory new and trimmed to consistent length, 15 pieces of virgin unloaded, and 31 loaded with my load that I worked up. (Yupp, 4 pieces of spendy brass were sacrificed to the pressure search, ending up with stretched primer pockets) That's where the question arose...Will the once-fired cases perform close enough to what they did as new, or is this load only really valid with virgin brass?
My assumption is (hunting rifle) not enough to make a significant difference, but I just don't know how much new brass really changes after the first firing. I guess I need to run a little experiment of the same load side by side in virgin brass vs the once-fired and see what happens.