Quote Originally Posted by lysander View Post
Why do people insist on saying "5.56mm NATO chamber," as if all things that were designed to shoot 5.56mm NATO have the same chamber?

The M249 has a difference chamber, and shoots 5.56mm NATO.
The M27 (HK416) has a different chamber and shoots 5.56mm NATO.
The Stoner 63 had a different chamber to the M16, although it was gone by the time 5.56mm NATO came along.
etc
etc.
NATO small arms Standardization Agreements (STANAGS) cover ammunition and links (9mm, 5.56, 7.62, and 12.7). They cover the drawings, dimensions and weights, and "Electronic Pressure Velocity and Action Time" (EPVAT) of the ammo.

Generally speaking, these came from WWII experience where the United States ended up making most of the ordnance used by the Free World and communist countries we were allied with to destroy the Axis. STANAGS were to standardize American manufacture and supply in the Cold War to supply NATO partner nations, whatever the source of their small arms (domestic- or US-produced).

Chambering is as unique as the manufacturer to ensure it functions with NATO-spec ammo. Japanese 7.62 was downloaded to function in their service rifle, and British Radway Green 5.56 was downloaded to work in their abortion of a service rifle.

There are still no STANAGs for the M16 magazine, stripper, spoon, and bandolier.