A little confused on headspace using a Hornady Headspace Gauge Kit

[eyeshot]

I have measured .308W- LC/ fired 1x @ 1.630 (mil)
CBC/ 1x @ 1.628 (mil)
Rem/ 1x @ 1.629
Fiocchi/ new ammo @ 1.624

I checked my dial caliper against a Vernier caliper, inches to inches, and they agree. The above numbers are 3 case/ cartridge averages. The cases have been decapped but not resized.

SAAMI specs- 1.640 max and 1.630 min

It would appear the the only the LC is the only one at SAAMI min. of the three once fired. What am I missing here?

 

[troy fairweather]

Are the cases sized, with the new ammo fiocchi maybe cip spec not sammi or they measure at a different spot on the shoulder.

 

[eyeshot]

Are the cases sized, with the new ammo fiocchi maybe cip spec not sammi or they measure at a different spot on the shoulder.

Not sized, Troy. I guess I expected the fired cases to be somewhere between the SAAMI max and min.

 

[Metal God]

You’re not missing anything, it’s not uncommon to get different measurements with different headstamps . Especially if those cases were factory loads that you fired . Lighter charges may not blow out the case fully . It’s common practice to neck size only after first firing and fire it again . This results in the case being much more precisely fire formed to your chamber after the second firing .

It is also not uncommon to get different base to datum measurements when shot from semi autos .

Regardless, if you measure enough cases a pattern/consistency will emerge that you can base your bump off of . Pick the longest measurement bump your shoulders .002 and see if you can easily chamber that case if not bump another .001 try to chamber again , if not repeat Until your cases frely chamber .

Always remember the Hornady case headspace gauge is a comparator . You are not trying to find an absolute measurement but rather comparing one case size to another . You can base your sizing accordingly . SAAMI specs are irrelevant as it relates to custom sizing cases to your chamber , all that matters is your chamber . FWIW when I measure real headspace gauges with my Hornady comparator it actually shows the measurement is approximately .009 shorter than the actual gauges . These are comparators not actual tools to get an actual measurement

 

[eyeshot]

I think I'm getting it. So . . the brass I have, as you can see above, is a mixed bag. I bought the Fiocchi (all I could find) to fire and use as a basis to size the other 1x fired brass.

The used brass was shot through my rifle, before I bought it, or through my brother's A10. So I don't know what was fire formed in my Savage 99 .308.

Thanks much for you and Troy's response. I asked these questions on a previous thread a while back but still wasn't getting it. Clouds are starting to break up. What can I say, I'm left-handed.

 

[MEHavey]

The Hornady gauge is a comparator -- not a pure/absolute headspace gauge.

Use it w/ a couple of unfired commercial cartridges* to calibrate "most probable"
true headspace dimension as read by the gauge. In this case it would appear the
gauge might appear to be reading several thou under. Write it down somewhere.
(The only real calibration would be if you had an actual SAMMI gauge to use the
comparator on)

The real use of the gauge is to:
- Determine where your rifle will just close (slight "feel") on a case.
- Size to 1-2 thousandths below that number.


* (likely manufactured for MIN headspace)

 

[Walkalong]

https://www.thehighroad.org/index.php?threads/case-gauges.759018/

 

[AJC1]

You’re not missing anything, it’s not uncommon to get different measurements with different headstamps . Especially if those cases were factory loads that you fired . Lighter charges may not blow out the case fully . It’s common practice to neck size only after first firing and fire it again . This results in the case being much more precisely fire formed to your chamber after the second firing .

It is also not uncommon to get different base to datum measurements when shot from semi autos .

Regardless, if you measure enough cases a pattern/consistency will emerge that you can base your bump off of . Pick the longest measurement bump your shoulders .002 and see if you can easily chamber that case if not bump another .001 try to chamber again , if not repeat Until your cases frely chamber .

Always remember the Hornady case headspace gauge is a comparator . You are not trying to find an absolute measurement but rather comparing one case size to another . You can base your sizing accordingly . SAAMI specs are irrelevant as it relates to custom sizing cases to your chamber , all that matters is your chamber . FWIW when I measure real headspace gauges with my Hornady comparator it actually shows the measurement is approximately .009 shorter than the actual gauges . These are comparators not actual tools to get an actual measurement

He Nailed it. The only thing I do different is start sizing backed off so the cases actually grow so I can find max length. After I find what max length is then I dial in shoulder bump.

 

[eyeshot]

https://www.thehighroad.org/index.php?threads/case-gauges.759018/

I looked at the "library" and totally missed that thread. In my mind I was looking for headspace. When I used the search engine I typed in headspace and I don't remember that popping up. Headspace on the mind, I guess. Thanks, for the link.