What kind of testing can be done with brass before it's fire formed?

You can do some, even if fl sized the powder capacity will mostlikly be different from New brass. Neck sizing will give the most powder space.

With rare exception, I always run a medium-range load through my brass before I start working with it to develop any accuracy load. I want to duplicate what the brass will be like going forward after it's fired. That's just me, however.

Walkalong said:

New correct for the chamber brass? And you are going to be FL sizing? Test away. Then match the neck tension of the new brass.

Or FL size it with your dies before the first firing. Even better. And of course I would trim/chamfer/deburr after sizing.

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what is FL sizing?

The consulting firm I work for (Part time now) use lots of new brass in various calibers by different manufacturers. We make an initial quality check of each new lot, using a quick test method for checking head squareness. Most lots check out OK, but not always. We once returned 2500 cases to a well known and respected maker.

FL = full length
Recommended for semi-autos like an AR

The only time I neck size is when the brass will stretch and give a case head separation in short order if I full length size. Mainly for my 303 BRIT. The rest of the time I will at least set the shoulder a couple thousands by partial full length resizing.
I will use new brass after running it through the sizing die to work up an initial load but then use fired to fine tune my loads.