I have measured the case volume for about a dozen .223 cases and found large differences ( up to 2 gr ) between same headstamps, different stamps, etc. I noticed the same thing with about a dozen 5.56 cases also. In .308 cases, the difference was as high as 4.8 gr. This is with the cases trimmed to the same length. My question/concern: Unless you physically measure each case, can't you put yourself in a position of overcharging?

Yes, as each BRAND of case will have different volumes, there is a risk of excessive pressure in one BRAND OF CASE OVER ANOTHER. This is why ALL manuals recommend keeping like brands together and not mixing brands when reloading.
Pistol cases seldom differ much in case volume, but rifle cases are a different story.
I have run across this myself, some loads are quite safe in one brand, but the same load in another brand is way over max pressure.
It pays to keep the same BATCH together also, because volume can differ between lots in the same BRAND.
Hope this helps.

Cheers.

This one more reason why we segregate case according to weight, after they have been trimmed and prepped.

What did you measure your cases with? Water is a viable substitute to the powder you are going to be using. Or you could try the powder instead of water. Have these cases been fired? Each cases will not stretch the same after each firing. If you are talking a couple of grains I would not sweat that. Have not found any brass that is absolutely perfect time after time. I would not load past your recommended max load, but you probably know that.

We select the same weight brass - normally within 1 grain.

This is done after the cases all have been trimmed, flash hole deburred and primer pocket uniformed.

After each firing, we find that some increase in length more than others - some actually become shorter!!

What Saeed said.
You don't have to fill the cases with anything to determine relative volume; just weigh them.
I don't; just segregate them by maker, and by age. Sometimes brass will change over the years for no reason.

Depending on how you measure the volume you can easily get that variation remeasuring the same case.

I measure volume to give me a data point for QL. In the past I would weight and sort all my brass. After testing I determined I was wasting my time on hunting rifles. So I sort by manufacturer and number of firings. Take the saved time and do other things.

Okay, I measured the volume using ball powder and lightly brushing it from the top with a piece of paper at a slight angle trying best to not further push powder down into the case. I measured various headstamps and also cases of the same headstamp/markings. Even same Mfg with same markings varied. All brass was fired at least once. All brass was trimmed the same and to the same length. Basically what I'm saying is that brass of the same lot varies. Now, I will say the biggest differences were between different Mfg'. In the case of .308, IAI measured 48.6gr and SSA was 53.5gr. Anyway, I appreciate the responses and while I was thinking about no longer sorting brass by Mfg, I think I will continue doing so. I think I might start comparing case headstamp/markings in regards to both volume and case weight. I mostly am concerned about overcharging but I've also identified specific Mfg's that hold lesser volumes and set those aside to load separately. I'm somewhat new to reloading and if I error, I want to be on the safe (anal) side. Again, thanks for the info.

Better safe than sorry.

I'll waive as I go north Sat morning up to Holbrook. Can't wait to leave this heat and get back to CO.