Jepp2,
That can be due to how the exact load times itself with the gas system. Also, have you set your fired cases in a straight line on a flat surfaced and sighted down them? If the case mouths and necks don't line up perfectly, there's a good chance some rim bending or burring has occurred that can mess with readings.
People who shoot bolt guns and neck size-only, find that after a number of load cycles that depends on how hot the load is, the case starts to get too snug to chamber freely and they have to go through one cycle of full-length resizing before starting to neck size again so the case will fit. What this tells you is that brass springback after firing is significant. If you use a light load, in particular, the case may come out already about as short as you'd want to resize it. This is why some people have gotten away with neck sizing-only even in some gas guns (though I recommend against it because of the added slamfire risk).
To know the length of the chamber itself, you'd want to measure a fired case, neck size it and shoot it again, and measure again and keep repeating until you felt it start to get snug, at which point it is actually slightly bigger than the chamber. This is possible because the chamber itself expands during firing and the steel is more elastic than the brass, so that when the brass comes right up to chamber size it thereafter is expanding with the steel, but not springing back quite as much. You then look back a measurement or two and have the actual chamber length. Plotting the past measurements in Excel to find a low R² trendline would be a good way to smooth the measurement errors out.
For the gas gun, for which I think neck sizing-only is risky, and to also to save range time, Mehavey's method is actually easier, though 1/16 of a turn on a standard die is 0.0044" inches, so I would use about a quarter of that per adjustment. I've attached a scale for this and for seating stems. The upper left one is what you want. Cut out the center and slip it over the die and onto the lock ring or the press (if you're not using the lock ring), then use a pencil or Sharpie mark on the die body threads as an index to see how many graduations you are turning it.
I have also found that not adjusting the die at all, but just running the brass back in and out of the sizing die can knock a thousandth off. Running it back up into the die and letting it rest a few seconds can sometimes get you up to a couple of thousands. So I would measure and test then re-resize and measure again before and try it in the chamber again before adjusting the die down at all.
Slamfire,
Beeeutiful pix! Thanks for sharing those.
A note to beginners: If you don't know what an incipient head separation looks like, look at the third photo, fifth case from the left (next to the beheaded case) and look at the line just above the head. That case will come apart along that line if you try to use it again (assuming it even survives resizing).
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Originally Posted by Slamfire
Incidentally, this lubrication prevented case head stretch.
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If you've looked through Varmint Al's pages, his FEA of the .243 shows the same thing will happen with dry cases if you have a smooth polished chamber. The stretch is not actually eliminated, but rather is then spread out over half an inch to an inch of the case instead of being localized at a pressure ring just in front of the head web. It would take an awful lot of reloads to thin brass significantly over that great a length.
Al also showed the resulting increase in bolt thrust from polishing the chamber was insignificant. Same with using petroleum-based lubes. You have to go to very, very low coefficient of friction lube, like molybdenum disulfide or hex form boron nitride (about 1/10 the COF of petroleum based lubes) to increase bolt thrust about 50%. Lots of old wives tales are out there on this subject.