Is this a realistic goal for ammo fired in a semi-automatic platform? I definitely don't doubt your process I'm just curious. Is fire forming brass to a chamber in an AR platform going to cause issues in feeding reliability?
Bolt gun resizing and "fireforming" don't apply to gas guns. Totally different game. The first reason is, the cartridge extracts, moves, while there is still "residual" pressure in the barrel. This was measured on the "M14" gas system
What you can see is the mechanism unlocks while there is still pressure in the barrel, but the pressure is below the rupture strength of the brass. This residual pressure is used to push the case out. In theory, the extractor only holds the case on the case mouth, it should not have to extract a case, the case should pop out under the pressure.
this is from Volume IV, Design Analysis of Automatic Firing Mechanism, by LTC George Chinn.
http://www.ibiblio.org/hyperwar/USN/ref/MG/
This extraction under low pressure (LT 650 psia) has the effect of stretching the case sidewalls, if case to chamber friction is high enough. And even with lubricated cases, which gave me zero case head separations in M1a's and Garands, I saw shoulder movement.
My lubricated cases were extracted while there was still pressure in the barrel of my M1a. When the case moved as the bolt moved, there was enough pressure to move the shoulder to the chamber walls, which of course, changes the shoulder shape. And that made the base to shoulder distance "long". I would not see this in a bolt gun as by the time I release the trigger, barrel pressure is zero.
And this shoulder movement in a gas gun will create strange OAL variations in ejected cases.
Is .004-.005 oal variation going to cause any real accuracy difference with commercial surplus military brass (relative to other potential variation inherent to mass produced brass?) Im also always well within max saami length for .223/5.56 brass.
For gas guns, the case should always be sized smaller than the chamber, I use small base dies where I can get them, I push the shoulder back about the same as you. Sometimes small base dies won't size the case to factory dimensions which is why roll sizers exist. Trying to "neck size" or partial neck size cases to be used in gas guns will only result in failure to extraction at some point in the future, as work hardening will assure the case will stick in the chamber.
In so far as accuracy, I am confident variations in powder pressure curves, bullet weight and centers of gravity. and primers, are far greater than case OAL. I am going to claim that primer variations are a huge black hole of ignorance. Only ammunition manufacturers with "primer mules" know the energy variations between primers, and primer lots. Primers ignite the gunpowder, the energy output, the mass ejected, the flame intensity, the distribution of all these with respect to time, is extremely important to consistent ignition. But, this is a huge black hole in the knowledge of the shooting community. Shooters just assume a primer is a primer.
Just shoot enough 22lr in bullseye competition and feel a weak round, and then look through your spotting scope and see the bullet landed no where near your break. Given enough of these to establish a pattern and you will figure out weak ignition is bad for consistent groupings. In F Cl;ass smallbore, you will see things like this:
You could say that the bullet drop was due to not enough powder in the case, but have enough of these drop outs, and misfires in pistols, and I am going to claim that 99% of these drop outs is due to poor primer cake distribution in the rim, causing weak ignition.
