For rifle, you need brass from the same maker, same lot number, with the same history. This means you start with factory new brass, or buy once fired brass that came form a single source. (Difficult to find.)
Brass varies quite a bit from maker to maker in terms of volume and neck tension. And it changes as it gets fired, so you can't mix fresh brass with brass that has been shot 10 times.
I generally start with 500 to 1000 new cases and sort them by weight, putting them in 50 rounds plastic boxes. Once I start to get split necks ( 10 to 20 firings, depending on the brand of brass) I toss the brass and start over.
For pistol I mix it all up and shoot.