Pay attention to the Lyman gauges, every one I have used has the diameter of the recess for the rim cut too small to accept a case that runs to the upper end of the dimension range, .378 inches.

Brass can be left long for a particular rifle.  I think it's a terrible idea for AR's and similar rifles in case the ammunition is needed for other rifles in an emergency.

Generally the "headspace" context in reloading is the case dimension from the case head to a datum on the shoulder.  It's obviously related to chamber headspace of the firearm, but is not the same thing.

We also use headspace gages to measure that dimension in order to insure the finished cartridges will function in the rifles chamber.  There are two types of gages, fixed gages that cause the sizer die to be set up to produce a "correct" length determined by the gage manufacturer, usually 0.008 inches (shorter than the chamber).  The other type of gage is a comparator that measures the length of the case so the sizer die can be set up to produce a custom case headspace, say from 0.0015 to 0.003 inches.

There are at least two motivations for using comparator dimensions.  One is to limit stretching of the case upon firing and extending its life, and the other is to improve accuracy, both while shortening the case enough to insure function in the firearm, easy chambering and extraction of an unfired cartridge.

All that is a separate issue from trim length.  Loaders that use Dillon equipment commonly trim every case to the maximum length on every loading, and some others do the same thing with their power trimmer of choice.  Others trim about 0.005 inches shorter than maximum to simplify trimmer set up; that is where I fall, mostly.  Others trim to 0.010 or even 0.020 inches shorter than the book maximum length, and that's fine, that's a choice they get to make.  In the case of .223 Rem cases, the necks are already short enough for good bullet pull out, so I am not an advocate of trimming them much shorter than maximum.

Allowing the case length to grow is a safety problem, and it doesn't occur due to the length of the leade, it's in the transition from the case neck to the leade.  That location is the "second shoulder" between the neck and the freebore .  A case neck that protrudes into that shoulder is restricted from expanding on firing, leading to high pressure.  The leade is the transition from the freebore to the rifling.  There is a good drawing at this site:

https://bisonballistics.com/articles/an-introduction-to-rifle-chambers

I'll add a drawing if I can find one I can link.

That's the bottom line, the question has been explored and answered, and hopefully cleared up a question about dimensions for another member.

I'll mention that using flat base bullets for this measurement is not sufficiently accurate due to the radius at the base.

I've let this thread run long enough.

I don't recommend not trimming 223.5.56 cases to SAMMI spec.

OP can do as he pleases.

We have a lot of new reloaders reading these threads and we want to teach correct reloading procedures here.

Thread is over.