Tech Note

May 21, 2001

TECHNICAL NOTE 45:  5.56 NATO vs SAAMI .223 REMINGTON CHAMBERS

BACKGROUND:
We are often asked whether our rifles feature NATO (North Atlantic Treaty Organization) or SAAMI (Small Arms and Ammunition Manufacturers Institute) standard chambers, and whether it makes any difference.  

FACTS:
.223 Remington (SAAMI standard) and 5.56mm (NATO standard) rifle chambers are almost identical.  The difference is largely limited to the “freebore,” the cylindrical space in front of the case mouth, and the “lead” or “leade,” the tapered region that eases the bullet into full engagement with the rifling.  NATO and SAAMI cartridges can normally be used interchangeably with no problem.

The SAAMI chamber features a less freebore and a tighter leade, which normally provide better bullet fit and match-grade accuracy than the NATO chamber.  It is wonderfully suited to match bullets.  ArmaLite uses a SAAMI chamber in all ArmaLite and Eagle Arms match and hunting rifles, and most of its M15A2 and M15A4 standard grade rifles. 

Millions of rounds of NATO ammunition have been fired safely in Eagle Arms' and ArmaLite’s SAAMI chambers over the past 15 years.

Occasionally a non-standard round (of generally imported) ammunition will fit too tightly in the leade, and resistance to early bullet movement can cause elevated chamber pressures.  These pressures are revealed by overly flattened or powder stains that reveal gasses leaking around the primer. 

The first few rounds of ALL ammunition, from whatever source or lot, should be checked for pressure and other signs of defect before firing large quantities.  If you have a problem, you can generally bet that the ammunition meets neither SAAMI or NATO specifications.

All ArmaLite 5.56mm barrels bear markings on the top of the barrel in front of the sight base.  If no marking is present the barrel is a .223 Remington SAMMI standard barrel.  ArmaLite has adopted a practice of using SAAMI chambers in its stainless steel match barrels and the NATO chamber in moly (phosphated) and chrome-lined barrels.

ArmaLite’s larger AR-10 rifles are all chambered with 7.62mm NATO chambers, so no marking is used.  .308 Winchester (SAAMI standard) ammunition functions perfectly in the 7.62mm chambers.

© 2001 ArmaLite, Inc.