5.56 datum line to case head dimension

Tex S · November 13, 2014, 06:47 PM

I'm trying to avoid buying a headspace gauge for the 5.56 ammo I will soon be reloading, as I already have a set of Hornady headspace bushings, so with the right data I should be able to tell if my ammo is within specs.

The correct bushing measures .330. Apparently that is the same dimension as the datum line on this cartridges shoulder. Does anybody know the correct length from that datum line to the bottom of the case head?

Maybe a "go" number and a "no go" number?

A quick Google pulled up "go" at 1.4636" and "no go" at 1.4736" . Can anybody confirm that this is correct?

HJ857 · November 13, 2014, 11:01 PM

There is a problem with the Hornady headspace bushings. They are comparators and not an absolute gauge in terms of SAAMI spec.

I and two of my buddies have the same Hornady system and none of them match each other and mine has the most deviation. I mean - not even close.

So, using the Hornady gauges may not match true case spec.

However, if you use YOUR gauge, on YOUR brass and apply those numbers to set up YOUR sizing die, then it's all cool. It doesn't ultimately matter if your numbers do not match SAAMI numbers as long as the relative numbers are ok (relative being fired case headspace vs. resized case headspace).

So what you do with that gauge set is to measure brass fired from your firearm, decide how much you want to set the shoulders back (.001 - .004" probably), set up your sizing die and test until you get the correct case size, as measured by that same Hornady gauge and you're done.

Basing your resizing off your actual chamber dimension or fired brass dimensions is generally going serve you better than worrying about SAAMI spec anyway.

rg1 · November 13, 2014, 11:15 PM

Saami drawing shows 1.4666" with a tolerance of minus .007"
Here's the drawing with the measurement at the .330" dimension. The Hornady .330 insert is a good tool to compare your fired cases and adjust your sizing die to only push the shoulder back a measured amount.
http://www.saami.org/pubresources/cc...0Remington.pdf

Bart B. · November 14, 2014, 10:37 AM

If you can zero your caliper to read 1.4636" on a .223 Rem GO headspace gauge with the Hornady gauge on it, then it'll read real numbers measuring cases.

As others allude to, just set your fired cases' headspace back .003" for semiautos, .002" or .001" for bolt guns.

You can add .001" to fired cases' headspace number to get a very good idea of what the chamber headspace is.

November 13, 2014, 06:47 PM	#1
Tex S Senior Member Join Date: November 12, 2008 Location: Fort Worth, TEXAS Posts: 909	5.56 datum line to case head dimension I'm trying to avoid buying a headspace gauge for the 5.56 ammo I will soon be reloading, as I already have a set of Hornady headspace bushings, so with the right data I should be able to tell if my ammo is within specs. The correct bushing measures .330. Apparently that is the same dimension as the datum line on this cartridges shoulder. Does anybody know the correct length from that datum line to the bottom of the case head? Maybe a "go" number and a "no go" number? A quick Google pulled up "go" at 1.4636" and "no go" at 1.4736" . Can anybody confirm that this is correct?

November 13, 2014, 11:01 PM	#2
HJ857 Senior Member Join Date: January 9, 2007 Posts: 447	There is a problem with the Hornady headspace bushings. They are comparators and not an absolute gauge in terms of SAAMI spec. I and two of my buddies have the same Hornady system and none of them match each other and mine has the most deviation. I mean - not even close. So, using the Hornady gauges may not match true case spec. However, if you use YOUR gauge, on YOUR brass and apply those numbers to set up YOUR sizing die, then it's all cool. It doesn't ultimately matter if your numbers do not match SAAMI numbers as long as the relative numbers are ok (relative being fired case headspace vs. resized case headspace). So what you do with that gauge set is to measure brass fired from your firearm, decide how much you want to set the shoulders back (.001 - .004" probably), set up your sizing die and test until you get the correct case size, as measured by that same Hornady gauge and you're done. Basing your resizing off your actual chamber dimension or fired brass dimensions is generally going serve you better than worrying about SAAMI spec anyway.

November 13, 2014, 11:15 PM	#3
rg1 Senior Member Join Date: November 6, 2001 Posts: 1,125	Saami drawing shows 1.4666" with a tolerance of minus .007" Here's the drawing with the measurement at the .330" dimension. The Hornady .330 insert is a good tool to compare your fired cases and adjust your sizing die to only push the shoulder back a measured amount. http://www.saami.org/pubresources/cc...0Remington.pdf

November 14, 2014, 10:37 AM	#4
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	If you can zero your caliper to read 1.4636" on a .223 Rem GO headspace gauge with the Hornady gauge on it, then it'll read real numbers measuring cases. As others allude to, just set your fired cases' headspace back .003" for semiautos, .002" or .001" for bolt guns. You can add .001" to fired cases' headspace number to get a very good idea of what the chamber headspace is.