T. O'Heir said:
BNIB brass is not ready to load. It needs to be checked for length(usually ok though and doesn't need trimming.), FL sized and the mouths of rifle brass need to be chamfered as a minimum. Even over priced Lapua and Prvi.
"...exactly as it would be when a manufacturer loads new ammo..." And it gets FL sized.
Now you're just making things up. If you had ever actually seen NIB Lapua brass, you would know it comes chamfered by the factory. If you had ever bothered to look it up, you would know Prvi is an economy brand from a former Soviet Bloc country (Serbia) whose component brass is typically less expensive than Federal, Winchester, Remington, or other domestic brands. If you think a high volume manufacturer of new brass final sizes it twice, then you may believe they run all their finished bullets through an additional sizing step before using them, too.
Nobody in mass manufacturing wants to pay for duplication of effort. High volume manufacturers are not using reloading type equipment so there is no resizing step built in. The brass comes to the loading machine with forming lube already washed off of and, unless an annealing stain is being left, it is already polished and they have no motivation to repeat lubricating and sizing and cleaning and have extra tooling to wear out and extra process costs.
I prefer measurements to speculation. I happen to have collected a lot of new bulk brass in .308 Winchester from different sources for different experiments. Some are just small samples of 25 or so, while some are 1000's. I went to the cellar and measured samples against a Clymer .308 GO gauge of new and never loaded or fired brass from Starline, IMI, LC, Winchester, Remington, Hornady, Federal, Sig Sauer, Norma, and Lapua. None exceeded SAAMI maximum case length of 2.015". None exceeded the SAAMI maximum head-to-shoulder datum maximum dimension of 1.634". All but one brand had that dimension shorter than SAAMI minimum chamber length of 1.630" by one to five thousandths (and yes, that minimum chamber is shorter than the maximum case dimension because SAAMI allows that there is room in a minimum chamber for a case to expand under closing force from the bolt, though that is not something I would use with a self-loader). Only the Hornady Match brass was longer, with maximum values of 1.632".
So what you have is brass that is just fine to load and fire as-is. Glen Zediker reported in his book, Handloading for Competition, that he loads and shoots Winchester .223 factory-primed brass on the brass's first pass. Can you shorten brass more with a resizing die? Yes. Brass has spring-back, though, so those dies are made to over-resize a little to allow for that. But when the case isn't fireformed to begin with, that just makes it even smaller and looser in the chamber and it works the brass unnecessarily. It is better, in that instance, to just handle mouth dents by one of the methods already described. You can chamfer if you want to. Most factories don't do it, but they do make sure the trim saw marks are burnished by tumbling.
So, when would you want to resize new brass? The Hornady Match brass I measured would be an example of an occasion for sizing smaller if you were running it in a self-loader, but it will still function in a bolt gun, and until all your brass has been fireformed once in your chamber, using a sizing die will not uniform it very well.
The bottom line here is to measure what you've got and decide for yourself if you want it even smaller or just want to fireform it before you start messing with it. Personally, I go for the latter. If I don't think I can trust the new brass in a match due to variation, it gets loaded for practice or plinking on the first pass.