Tom has it right. Back when the M14 was still king at the service rifle matches, a lot of guys would trim the .308's about 0.015"-0.020" short on purpose. The idea was that since those cases grow an average of about 0.005" per firing, they could trim once and never have to trim again within the life of the case (which was usually only 4 to 6 reloadings in those guns). And these were match shooters, so you can see any affect on accuracy was negligible.
My concern is that they will weigh much less by being shorter, so I won't have an accurate idea of the case volume.
It's not relevant if you don't crimp your bullets, as long as you seat to the same COL as you did before, the usable powder space under the bullet will be unchanged, so your loads will be unchanged.
Cartridge brass has a density of 8.53 gm/cc. 0.013" of brass tube that is .308" on the inside and is about 0.014" thick will weigh not quite 0.4 grains. That is smaller than the manufacturing difference of one case to the next for all but maybe Lapua or Norma brass. That's 0.047 grains of water capacity and will change your loads by approximately 0.023 grains if you use the same COL. It's a smaller change than you can measure on a standard powder scale.
If you do crimp your bullets in a cannelure, then setting a 180 grain bullet over 4064 back by 0.013" is the equivalent of adding 0.1 grains of powder. It raises pressure about 400 psi and velocity about 5 fps. In other words, again, this is within the normal range of error for loads.
An equal concern is that they will grow too much and I might run into unpredictable case life due to case/head separation.
I am keeping these ones separate for my other lots. Thanks for the response.
The growth is governed by the amount of excess headspace. For the .30-06 that's determined by the length from the base to the shoulder of the case, subtracted from the length from the base to the shoulder in the chamber. The growth itself happens when pressure is high enough to stick the case to the chamber wall, then stretch the brass at the junction of the case head and wall to set the head back against the bolt face. It doesn't happen in the neck. (It also doesn't happen in cartridges that operate at pressures too low to stick the brass to the chamber—those operating below about 30,000 psi peak). So, trimming the neck has no effect on that growth.
The neck itself grows during resizing. This is because brass flows from the shoulder into the neck when the sizing die squeezes the shoulder back to a shorter position. The excess brass has to go somewhere.
SAAMI has a maximum case length to keep people from jamming the necks into their chamber throats, which can raise pressure dangerously. The minimum exists mainly so that bullet makers know where to put crimp cannelures when their bullets are seated to their design seating depths. So, its mainly an exercise in manufacturing coordination. If you took way, way too much off the neck, say, a tenth of an inch, then you might start to see some difference in bullet grip that could affect start pressure enough that mixing the short and normal length cases together could cause groups to open a bit.
The bottom line is that maximum case length is a safety critical dimension while minimum case length is a non-critical dimension.