NATO vs. SAAMI chambers

[Hoser23]

I read somewhere that using 5.56mm NATO ammo in an Armalite (SAAMI) chamber can result in increased and possibly excessive pressure. Although I was under the impression that there was no practical difference between the two, the source indicated that the cause is some slight differences in the dimensions of the cartridge cases. Armalite's website states that the problem does not exist, but I would like to know if anyone outside of the factory has experienced this problem.

[Epinephrine]

They have basically the same exterior dimensions and length, but the 5.56 is loaded to produce higher velocity and chamber pressures. It has thicker case walls for the extra strength needed to handle those higher pressures. THe difference comes in the chambering of the rifle, and that difference is the "Leade" (more commonly known as the throat) which is the portion of the barrel directly in front of the chamber where the rifling has been removed to allow room for the seated bullet. In a 5.56 chamber the throat is typically .162" -in a .223 chamber is usually .085" (about half the length). The result of firing a .223 cartridge in a 5.56 chamber is a slight loss of velocity when measured against the 5.56 cartridge. If you use a 5.56 cartridge in a .223 chamber rifle, the rifle will have a very hard time extractign the cartridge if it can extract it at all. Plus the excess pressure in the chamber can cause the rifle to blow up, but no recorded cases exist.