mehavey:
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Does the firing pin strike actually set the shoulder back that significant amount...
and therefore primer backout is not a measure of excess headspace?
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Yes, the firing pin does that. Do you own tests as I described.
Primer back out happens virtually all the time. It gets pressed back in when peak pressure pushes the case back against the bolt face. I've shot reduced loads in 308 Win cases more than 10% below max and extracted cases have their primers backed out several thousandths as well as case headspace being shortened a few thousandths more than normal. Further reduction of loads ends up with the primer sticking out even more. Increase the loads back to maximum and the primer gets reseated deeper. Interesting. Fascinating. I did 1 grain reduction increments in .308 Win. charge weights from 44 grains of IMR4064 under Sierra 168's all the way down to 35 grains. At 39 grains, ejected cases had primers protruding past the case head and fired case headspace was less than before firing. At 35 grains, primers stuck out almost .007." This was with Federal nickel plated cases; very slippery compared to bare brass ones whose shoulders didn't set back quite as far; .002" to .003" per pin smack.
06shooter:
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What is the diameter for .308 in a barrel ( lands across one another) ?
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That diameter is the bore diameter shown in the SAAMI picture above for the .30-06 case. .300" is normal.
One other thing I forgot to mention that adds another variable to the actual bullet's jump distance to the lands. With the full length die sizing down a fired case, the distance from the case head to the shoulder will vary a few thousandths. That's a normal spread caused by a combination of the lube on the case and how much spring the press has. So, even though a bunch of cases have their bullets seated to an exact distance from their ogive back to the case head, that spread in head to shoulder dimension (case headspace) will have a few thousandths spread. The distance from the case shoulder to that point on the bullet's ogive will have that same few thousandths spread. Measure a bunch of sized cases and see what the spread is.
When the round fires, it's hard against the chamber shoulder and that spread changes the bullet's jump to the rifling whatever its amount is. The end result is, with all the variables at hand, an exact distance from head to bullet point ends up with a few to several thousandths spread in the bullet's actual jump to the rifling.
The reality of all of this is, if you want the most consistent bullet jump to the rifling, the reference point on the bullet must be equidistant from a reference point on the rimless bottleneck case shoulder. How far away the case head doesn't matter; it'll vary a small amount.