Originally Posted by Sig 210

There are several myths concerning military 5.56mm ammo. It is a myth that US made 5.56mm cases are thicker than commercial cases. For over 40 years I have weighed my cases when real good accuracy is desired. See the link below for case weights.

It is also a myth that 5.56mm military ammo is loaded to much higher pressure than commercial .223 ammo. US military 5.56mm ammo is loaded to 52,000 Psi + 3 standard deviations; not to exceed 58,000 psi. SAAMI specs are 55,000 psi.

SAAMI pressures


BTW: There are at least a dozen different .223 chambers. Unless your rifle is an H@R Handi rifle or it has a tight .223 match chamber there are no problems with firing 5.56mm military ammo in that gun. H@R re-ground their reamers and their chambers are all over the place from huge to very tight.

A gunsmith friend says that he knows of no US firearm maker who currently uses a SAAMI chamber for the .223.

This is a good link to peruse:

AR15BARRELS.COM - Technical Documents