Lavid2002,
Wwmkwood makes another good point. I just assumed that your casehead shot included cases that had been loaded across the range of loads you listed, which would mean some of them had been fired at rather low pressure. If they are only from your last load, however, then they could be a pressure sign.
Pull the bolt and run your finger over the extractor to see if it stands too proud in the bolt face? I am out of town and away from my books, and I don't recall the dimensional tolerance. If you PM board member Walt Kuleck (username, wjkuleck), he can tell you, I'm sure. He is part of Fulton Armory which builds match guns and he is co-author of several books on arms assembly and disassembly, including The AR-15 Owner's Manual and AR-15 Assembly Guide. He could tell you most quickly how to check for a problem there? Assuming those marks appeared on the cases with the first load you listed, though, then they are happening without high pressure.
On case capacity, the reason it is variable is different internal case dimensions. SAAMI specs for commercial cartridge case brass are only for the external brass dimensions, leaving the internals up to the manufacturer to design. Military brass has both specified external dimensions and has to meet certain performance specifications, but, AFAIK, the specific internal dimensions needed to meet those requirements are still up to the maker. UNS C26000 70:30 cartridge brass has a density of 8.53 grams per cubic centimeter. If the external dimensions are fixed, it follows that a lighter case has less brass taking up volume on the inside than a heavy one does. The weight difference thus accounts for the difference in internal volume.
An example that illustrates some extremes is the .308 case. Winchester uses a semi-balloon head case design that weighs about 156 grains on average. It has about two grains more water capacity than a Lapua case that weighs about 171 grains, and three more grains of water capacity than an IMI Match case which weighs about 186 grains. The Lake City Match brass I have weighs about 180 grains. They all have the same external dimensions and the same size primer pockets, but their internal volumes are all different.
If you look at a high sample rate laboratory graph of chamber pressure measured by a strain gauge, you see a brief stall in the pressure rise at around 30,000 psi where the brass yield is surpassed and the case wall at the edge of the head experiences plastic stretch to let the case head move back to touch the bolt face, thus forming the pressure ring on the inside. This expands the volume the powder is burning in and lowers the peak pressure the chamber sees. Peak pressure in a case loaded to exceed about 30,000 psi is thus determined by the volume it fireforms to in the chamber rather than the volume it is sized to.
To help find your specific case volume, I have an Excel file in
my file repository to help. It includes instructions. Basically, though, you will weigh a fired case, then fill it with water level with the top (no meniscus) and weigh it again. The difference in the two weights is the case water capacity. My Excel file also calculates water density for the water temperature you use, and takes that small adjustment into account. It also asks you to enter the case length but the current version doesn't do anything with that information. It is just there to make a record to use in QuickLOAD which needs that to calculate the volume subtracted by the bullet at different seating depths. It also determines sized case capacity in a second column to be used to figure the point at which the powder loaded becomes compressed.
If you PM me with your fired case water capacity and length, that is enough information to give you a better pressure estimate for the 2015 and to recommend a starting and finishing load length that are specific to your gun.