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It's hard to answer your question because of the variations that are found with the tools, the chambers, and the cases.
There are two issues that complicate headspace measurements; the true diameter that the tool uses as a datum and the angle of the shoulder of the case. Both add errors in the actual measurement of headspace.
I have a friend who is a machinist/gunsmith and he made a HS gauge for me. His gauge is styled after the Hornady gauges but he used better materials (stainless steel) and milled the diameter to a true 0.400" with no taper to the gauge's contact point. A lot of gauges use a taper where they touch the shoulder of the case but I didn't like that idea, if there is any difference between the angle of the gauge's contact point and the case's shoulder angle it will give you a false reading. With no angle to the gauge it will always give a headspace reading at a 0.400" datum point, regardless of the angle.
SAAMI specs specify a shoulder angle of 20 degrees for a Winchester .308 and the shoulder angle spec for a 7.62x51 case is 40 degrees. Depending on chamber and case shoulder angles you will get varying degrees of accuracy when you measure the HS.
My guess is that the case's shoulder angle is less than the chamber's shoulder angle. If that were true then the case can grow a bit more before it's shoulder contacts the chamber's shoulder. That could be due to wear in the chamber or how it was originally cut. If the barrel has had hot enough loads and with enough time, flame cutting could have enlargened the front end of the chamber, including the shoulder. Considering how long your HS seems to be I think that this is a strong possibility and that basically you are looking at a shot out barrel.
There are two issues that complicate headspace measurements; the true diameter that the tool uses as a datum and the angle of the shoulder of the case. Both add errors in the actual measurement of headspace.
I have a friend who is a machinist/gunsmith and he made a HS gauge for me. His gauge is styled after the Hornady gauges but he used better materials (stainless steel) and milled the diameter to a true 0.400" with no taper to the gauge's contact point. A lot of gauges use a taper where they touch the shoulder of the case but I didn't like that idea, if there is any difference between the angle of the gauge's contact point and the case's shoulder angle it will give you a false reading. With no angle to the gauge it will always give a headspace reading at a 0.400" datum point, regardless of the angle.
SAAMI specs specify a shoulder angle of 20 degrees for a Winchester .308 and the shoulder angle spec for a 7.62x51 case is 40 degrees. Depending on chamber and case shoulder angles you will get varying degrees of accuracy when you measure the HS.
My guess is that the case's shoulder angle is less than the chamber's shoulder angle. If that were true then the case can grow a bit more before it's shoulder contacts the chamber's shoulder. That could be due to wear in the chamber or how it was originally cut. If the barrel has had hot enough loads and with enough time, flame cutting could have enlargened the front end of the chamber, including the shoulder. Considering how long your HS seems to be I think that this is a strong possibility and that basically you are looking at a shot out barrel.
This is a follow up to an earlier post. My M1A is stretching brass like crazy and I wanted to know why. Now I am really stumped:
My M1A's bolt won't quite close on a 1.638" headspace gauge (Forster .308 Win field gauge). The gauge goes easily into the chamber.
I used my RCBS Precision Mic (.308 Win) to measure both the headspace gauge and the brass that I fired in the rifle, with the gas valve shut.
The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.
Couple of details: Like new rifle probably has less than 500 rounds through it. New Black Hills Match ammo was used when shooting with the gas valve shut. The bolt was stripped, and all parts, chamber, and Precision Mic thoroughly cleaned before measuring. Headspace gauge is brand new and there are no burrs on the brass.
With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.
Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?