Trimming unfired brass

i recently picked up 100 rounds of Norma 7mm rem mag brass. I wanted to do as much case prep to ensure uniform cases. The question I have is that I measured headspace using a hornady headspace gauge and the rounds varied from 2.106-2.109 which is a .003 difference. If I were to trim the cases to a trim length of 2.490 wouldn't that mean that some of my necks would be .003" shorter than others? Because the difference in length is in the shoulder or case body and not the neck? I sorted the batch into three groups that had very consistent headspaces and went ahead and trimmed them to 2.490. Should I recheck them after fireforming and trim again or should I have waited to trim until after fireforming and the headspace measurements should all be the same...or am I overthinking this?

Thanks for the replies!

The first thing I would have done is to have measured the overall length of the chamber. Brownell/Sinclair has the gauge to do it. Just a pin you slide in a shortened case neck. Will work with factory SAAMI chambers. Once this is found, set your trim .010" short of max length.

Plus, I would have fired the new brass in your chamber before trimming. It often takes two firings to fully form to your chamber. Then, your base to datum length would have been practically identical. That's when I set up to trim, because SAAMI chamber neck lengths are notorious for being
long.

I'll usually trim my Lapua 6BR brass after a couple firings when a bit more of a length disparity shows up. I'll set the trimmer to 1.555 or 1.556, and the trimmer barely touches most of the brass.

Head space and trim length are different matters. You don't fix headspace with trimming.

I would make sure it fits the chamber before you load it, then after it is fire formed to your chamber, I would size it and then trim all of the brass to uniforme lengths as long as it was not longer than SAMME specifications.

As long as you have neck clearance between the mouth of the case and the end of the neck chamber your ok. There is no need to trim it .020 below SAMME specifications, In fact to much clearance can create space for carbon fouling.

I like .010 clearance so while firing, the case will grow .004 to .006 thousandths and this growth will not hurt accuracy or pressure.

To get consistent neck tension, It is important to trim all cases to the same trim length. the amount is not as important as long as they are not trimmed excessively.

Just my opinion

J E CUSTOM

I don't trim new brass.
Often, even with match grade/premium brass, you will see that the first firing will lengthen the neck unevenly, and sizing it will lengthen it even more unevenly.
It can take 3 neck sizings for the brass to stabilise and stop growing unevenly.

Bushing dies arrest this somewhat, but can cause donuts, which I hate.

I have never seen a difference on target with brass that had .003" in neck length difference.
Uneven necks have caused a flier here and there though.

I trim after the first firing/sizing and check after every firing for uneven neck growth.

Cheers.

Did you run them all through your dies first? I always do that. Not that might change it but it could narrow the spread.

The shoulder location on new belted cases can be all over the place because the belted case does not headspace on the shoulder. Meaning check the shoulder location "after" the case has been fired.

The sizing method and the OAL will vary depending if you neck size or full length resize with minimum shoulder bump.

I full length resize all my new cases to make sure the case body and neck is concentric and then trim to minimum case length and deburr. After the first firing the trimming will depend on case growth, sizing method, and if you wet tumble and ding up the case mouths, etc.

I run all new brass through a specifically sized/honed neck die with a polished/sized graphite lubed expander.

In my comp guns, after fire forming the brass, they get put through mandrels and turned, then final sized to my specs.
This appears to have the least amount of neck growth.
Expander balls appear to stretch necks in standard sized FL dies IMHO.

Cheers.

On new brass I run it through the FL die and inside-outside champhor the necks. I just lock my dial caliper at max. case length and pass the brass through it to check make sure none is over length. After the first firing I run them through the case trimmer.