Take a look in the reloading section at the thread titled "Weird things happening with LC brass". I agree there are no differences in exterior dimensions between .223 and 5.56 cases but one poster in that thread points out that a 5.56 NATO chamber has longer headspace limits than a .223 chamber. For my purposes, I consider the maximum HS to be 1.4736, the Colt Field II dimension, for both. I also have a .223 chamber drawing that calls out the max HS as 1.4736.
As to the only differences between the chambers being ahead of the case mouth, it looks like the 5.56 chamber is slightly fatter at the base (Ref C) and the diameter of the chamber 'neck' is slightly larger (Ref F) and the 'neck' length is slightly longer (Ref M).
Picture taken from Luckygunner writeup on .223 vs 5.56


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.
Last edited by Eric D.; 09-15-13 at 19:47.
B.A.S. Mechanical Engineering Technology
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