Well that got responses fast! All good arguments so far, can't find fault with any of them, all approaching from a different angle which is good. No one has argued for powder yet though it has been voted for. Here's my thought for now
in my experience powder has made more of difference than about anything else
yes the bullet is the critical element...but it's amazing to me how well I've managed to get visibly inconsistent bullets (Speer btsp especially, noticeably out of round, inconsistent weights...my 243 loves them!) to shoot out to at least 400 yards.
we've seen in the last 2 decades that match bullets can often be used effectively as hunting bullets
hunting bullets can be capable of match or near match precision at long range
even fmjs can kill quickly
the bullet can be worked around pretty easily to do what you need it to do, even if that isn't what it's ideal for.
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moving on to brass...feenix I agree we obsess about it and most of us who have done this a while end up spending money our former selves might have hit us with a stick over on "premium brass" because if we're serious about this hobby, it is worth it.
BUT...anathema as it may be, I've had great results in the past shooting loads with MIXED HEADSTAMPS!!!! It seems brass in this regard isn't nearly as particular as I'd like to think. Try shooting mixed powder, mixed projectiles, even mixed primer brands and I'm confident it makes more of a difference for the worst than not worrying about brass will. Absolutely I prefer some brass to other brands but there is no brass I can't make work pretty good if I need to use what's available.
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Primers as well....some yield better es and sd numbers, they all work, enough said. And I know you shouldn't but I've switched primers and kept loads otherwise the same and, well, nothing of note happened.
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My vote: Powder. There as some that just work sooooo much better than others for a given application. An improper burn rate shows little mercy, you will have to accept lousy performance if you cant find anything within a certain window, especially with magnum cartridges.