308 headspace gage check with micrometer case gage

[krxm21]

I picked up some 308/7.62NATO headspace gages.
I decided to check them in my MO Micrometer Fired case gage.
Green gage reads minus -.001 Red gage reads plus +.007
I'm thinking the Red one is actually a Field gage spec.

Anyone have a Field gage that they can check and report on?

 

[jscandale]

I would question accuracy of the fired case gage. You really ought to measure it with a high quality micrometer and a bushing bored to the correct diameter corresponding to the dimension on the saami print.

JS

 

[krxm21]

I would question accuracy of the fired case gage. You really ought to measure it with a high quality micrometer and a bushing bored to the correct diameter corresponding to the dimension on the saami print.

What do you think the difference between go and field should be? Based on 308 Win or 7.62?

 

[jscandale]

I think there is .006 between go and no go. The field gage might be .007 or .008 over go. I'm not 100% sure about the field length but it's close.

JS

Just found this:
These are for a 7.62 NATO chamber
GO: 1.633
NOGO: 1.6385 (+.0055 over GO)
FIELD: 1.6455 (+.0125 over GO)

I was close...
For a commercial .308 Win, you can subtract .002 from the NO GO value and about .006 from the FIELD value. For all intensive purposes, the chamber should be within .002 of the GO gage and the FIELD should not even be considered for a sporting arm. The FIELD gage was only a useful armorers tool to check excessive wear on the M14 that just needed to function safely in the field for a GI that was firing ball ammo from one of the many arsenals which pumped out millions of rounds of ball ammo for the troops.