This is something that always draws fire, but here goes...
Your basic CHAMBER is supposed to be SAAMI specification.
Since the chamber is *Supposed* to be SAAMI specification, the brass/loaded round is supposed to ALSO be SAAMI specification to fit that chamber.
This DOES NOT happen, every manufacturer has their own twist on chambers, mostly because of lawyers.
The chamber is intentionally built a little sloppy so it accepts & contains a wide variety of manufactured ammo SAFELY, and so customers don't throw a hissy fit when cheap/poorly made ammo won't fit...
Another reason in oversized chambers is the popularity of auto-loaders.
Lesser made auto-loaders need 'Slop' to allow rounds to chamber (and the reason 'Accuracy' guys often default to manual loading rifles).
When your chambers are 'Precise', the ammo also has to be precise to fit that chamber.
(See the long list of Ruger American loaders that have to 'Cam Over' presses to get reloads to fit the very same chamber, Ruger American having a precisely cut chamber that people call 'Tight').
There is a seemingly endless supply of 'Precision' dies that allow you to size the different case sections accurately, to your chosen specification.
From case sides (body), to Datum line (shoulder 'Bump' to fit chamber headspace), to neck size.
It's up to you what you want for your application, but be aware this is a rabbit hole you can get lost in!
Unless you buy up a LARGE lot of brass so all brass you use/size is EXACTLY the same composition alloy, and you have a PRECISE annealing process, the brass itself will change as it's used, creating a difference in the brass no matter how accurate your dies are.
For common consistency I suggest a die that closely fits the chamber you already have...
Let me say that again,
Find, or modify dies to closely fit the chamber you already have, and use those dies for that one particular chamber.
Common dies often WAY over work the neck of brass.
They deprime the brass, while fired brass has an expanded neck, the depriming pin and expanding ball enter the case.
The case neck is then forced into neck sizer, WAY undersizing the neck, working the brass too much, then as the brass is pulled out of the sizing die,
The expander ball is dragged through the neck further overworking the neck.
Honing out the neck of common dies keeps the over work to a minimum.
This die is made to mash down the THINNEST neck brass, so thicker brass necks get seriously overworked.
The idea is to open up the die neck just enough the expander ball contacts it for proper sizing, to retain the bullet in the case neck.
The converse is true also, when your bullets drop into the case, or you need a crimp to hold bullets in the case when chamberings the round, simply hone/lap down the expander ball a little to increase neck hold on the bullet.
This often elminates the need for crimping which can mess with consistency.
The dies are NOT sacred, it's perfectly acceptable to modify dies to specifically fit ONE chamber.
You don't want excessive wear on your fitted dies, and the rounds those dies produce might not fit other firearms in that same caliber correctly.
