Quote:
1. As brass is fired multiple times, it flows forward, thickening the neck and shoulder area of the case, lengthening it in the process.
2. As brass flows forward, it also thickens the neck walls, reducing the internal dia. When the round is fired, normally the neck expands to the limits of the chamber, allowing the bullet to pass forward into the throat.
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This ain't what I observed when I loaded and fired the same case 47 times.
Starting with a new Federal .308 Win. case trimmed to 2.000" length, a max load was put in then fired in a SAAMI spec chamber. The case shortened to about 1.997" and its neck was blackened with powder residue. There was clearance around the case neck when the bullet got pushed out into the rifling. Loaded round neck diameter was .337" and the chamber neck was .344".
Full length sized it and length grew to about 2.001". Reloaded it then fired it. Case length shrunk back to about 1.998"
Sized it again and length grew to about 2.002" After reloading and firing it, length shrunk back to 1.999"
Each firing, sizing, reloading and firing again did the same thing. Each time the neck was blackened with powder residue. When the case grew to 2.010" it was trimmed back to 2.000" and this happened every 9 or 10 cycles.