You, the reloader, have the option, of your own free will, to size or NOT size your new brass. That is totally up to you!!
I used to be the nay-sayer, pooh-poohed my friends who had the time to load their ammo and confirm it fit in their guns. Hah! I could get new brass and inside of an hour or so have ammo ready to shoot. Why spend the extra time?
Spend a heck of a lot of money on a hunting trip, or be tied with another competitor going into the last round of a competition, and then tell me what that VERY FIRST piece of bad brass is worth.
I load 1000s of rounds a year. Quite a bit of it is new brass or once fired brass that has been made into wildcat brass. Lots of my straight wall pistol brass is new when first fired in my revolvers. I should know what should or should not be sized, right?
A couple of years back I purchased a new .243 WSSM a few weeks before hunting season. I found one box of ammo locally for about $50. I could pick up once fired brass for $25 per 100. Yep, spent the $50 on once fired brass. Picked up my new dies. Set up my loader. Was very maticulous in setting up the die only doing a partial neck size until the brass fit in my chamber. After all, 200 once fired brass, all from the same guy, must have all been fired in the same gun, right? Partial size to fit my chamber and I won't have to over-work the brass too much.
I loaded up my ammo with known components to a load that would be adequate for my hunt. Using all top of the line components, it has to be accurate too, right? I sighted the gun in. Several 3-5 shot groups out to 200 yard. All under an inch, even out to 200. What a sweet shooting gun.
We got out to our hunting spot the night before season and were glassing the slopes around where we would hunt. Too early yet for the deer to be moving in, or maybe something else is going on. We look closer and a buddy of mine spots a large coyote about 400 yards out, laying in wait. He is right where our deer are supposing to be coming up. My buddy runs into the trailer and pulls out the first gun available. Of course is it my .243 wssm. He asks me quickly for sight picture and I say hold top of back level with crosshairs. He lets fire and Mr. Coyote jump as if stung and spins around. Grazed him. My buddy tried to jack in the next load. WHAT!! the bolt won't close. We get the round out and get the next in line. Mr. Coyote is just disaapearing over top of the ridge. No time for a second shot.
Of course we have to find out what happened. Why wouldn't the bolt close? We tried the round several times and finally got the bolt to close, but with a lot of pressure. I check the rest of the ammo in my box of 50 and found about 8 that required excessive pressure to close the bolt. I hadn't adjusted my die down enough to fully size each piece of brass for my chamber. Some of the once fired brass had to have been fired in a different gun. Of course I corrected that when I got home and sorted thru the rest of my ammo.
Factory brass is not made on dies that look like what we hunters and reloaders use. It is stamped out of rod or billet that is going thru multiple steps in stamping machines. These machines work better than 99.9% of the time. Loss is well less than 1 per thousand. More likely one per every 3000 or 4000 or more.
Talk to someone who works in the factory actually loading this new brass. In a typical shift they will load 10s of 1000s of rounds of ammo and will have a small bucket of reject brass that they pull out during the process. Brass is still going thru another sizing machine, priming station, charging station, and bullet seating. Some go thru a pretty funky looking machine that spins them to put in the crimp. Once in a while you even have rejects get thru at the manufacturing level.
If the brass isn't going into be loaded, it goes to be sold in bulk. Boxes with 2000+ pieces of brass dumped in by weight. Do you think they are sorting at this level? Nah, they are loading those boxes and getting them stacked on a pallet. Sometimes the wholesaler will package them in small packages, but even then they do not always do a good job of sorting.
I won't tell you what to do as far as your brass goes. It is your time and your reloads. If you load to go blast in the gravel pit, why waste the time? If you are loading for hunting or competition, you better think twice about your options. Your first BAD brass may be the first shot you HAVE to take.
Steve