My fault. I'm too wrapped up in one detail and got lost with everything else.
The rifle/ chamber in the immediate question is a "from the factory" Remington 700 .223 with about 800 to 1000 rounds through it. The only "smithin" done to the gun has been a trigger replacement. The gun will shoot some factory ammo to just under 1 MOA at 150Y and some of my reloads have shot 5 round groups to under 1/2 MOA. Later on I'll be looking at another 700 with a SAMMI spec'd barrel from Hart in 7mm Rem Mag. I'll look at this gun another time.
The .223 brass in question is 1x fired factory ammo from Remington - qty 50.
At this moment, the brass has been decapped with a decap die (not a sizing die), ultrasonic cleaned, primer pocket brushed clean, flash hole reamed with a Lyman flash hole tool. they were then Body sized with a Redding body Die and the shoulder moved back .002 to .0025 measured with an accurate Starret dial caliper and Hornady headspace gauge. The necks wall thickness' were then measured with a Redding Neck Gauge to see if any cases were perfectly concentric. None have a runout of less than one and one half thousandth of an inch. Most have runout between two thousandths to four thousandths of an inch. Four cases are over four thousandths and have been marked and isolated. I'll use them for fouling shots or the like.
As mentioned before, the neck wall thickness' range from .010 to .014+" the really bad cases were the thinnest.
The next steps anticipated are trim to length, deburr & chamfer. I'm waiting for a chamber length plug from Sinclair to determine the actual length of the chamber before cutting the cases. Then neck sizing before turning the necks before cleaning again. The last step to complete case prep will be to measure case concentricity.
The goal, if possible, is to create a batch af cases that are as concentric and identical as possible considering the stock. Then to explore what improves accuracy, and of course what shouldn't be done. If they are turned too thin and don't last but a few firings that will be OK. When I get an idea of what the rifle likes to shoot then I'll get some good brass and continue to squeeze every last bit of accuracy from the gun. I just don't want to damage the gun or myself for that matter, in the process.
Above, I've given the details I know at the moment. Is there anything else one would need to know? If so, I'll measure or whatever when I get home tonight.
Thanks again for the help
PS AS I'm writing this, I'm thinking about what you have already mentioned and I'm thinking I should have a chamber cast made in this gun or will a carefully handled fired case give me a sufficiently accurate diameter?