Originally Posted by
MK18Pilot
Putting manufacturing tolerances aside, there is no difference in external dimensions between a SAAMI .223 Remington and a Lake City M855 cartidge case. If you believe there is, I would love to see the numbers.
That being said, I'd argue that a headspace gage marked 5.56 NATO is nothing more than marketing 3-Card-Monty. The various chambers are only different in front of the case mouth.
I'm also calling bullshit on any manufacturer or retailer selling a gage that claims to be accurate to ten thousandths. That's an aerospace quality dimension and requires that the gage be manufactured and used at specific temperatures. Truncating the headspace dimensions at a thousandths would have zero impact on safety or function of the weapon.
To accurately gage head space, the barrel has to be removed from the upper receiver and both the extractor and ejector have to be removed from the bolt. It's nothing more than mental masturbation if you aren't.
If you just have to buy a gage and unless you are a barrel manufacturer, IG is correct that a field gage is all you need. A max/reject is preferable but they are hard to find.