When you fire a 5.56 M855 cartridge in a military chamber with its longer throat the chamber pressure is 55,000 psi.

When you fire a civilian .223 cartridge in a civilian chamber with a short throat the pressure is still 55,000 psi.

Meaning military 5.56 ammunition is not loaded to higher chamber pressures. "BUT" if military ammunition is fired in a civilian short throated .223 the chamber pressures will be higher. And the throats are what cause this change in pressure readings. And the European CIP considers the .223 and 5.56 cartridge to be one in the same.

The SAAMI interchangeability warning between the .223 and the 5.56 did not come out until 1979. And it was at this time the military introduced the M855 cartridge and M16 rifles with a longer throat.

The SAAMI sets guidelines for chamber dimensions and the gun manufactures are free to change chamber and throat dimensions as they see fit.

I can load my Savage .223 hotter than listed loads in reloading manuals because it has a longer throat than either of my AR15 rifles.

And the max load for the .223 with 55 grain bullets in the Hornady manual is 23.2 grains of H335. And the max load for 55 grain bullets and H335 in the Sierra manual is 27.5 grains.

I will take a wild ass guess and say the difference in these two .223 rifles is throat length. And a good reason why they say to start low and workup when reloading.

Now look below at all the different throat lengths and diameters. My off the shelf factory Savage .223 has a .0566 length throat the same length as the PTG and JGS NATO chambers. And its the .0250 length throats that will cause the higher pressures and possible problems. And not by people guessing about how the pressure is measured in what type chamber.


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