ar-15, 5.56x45, saami, nato, .223-remingtonThere seems to be some confusion and misinformation about the differences between these two rounds. Some say you can fire both in any rifle chambered for .223, while others claim that the higher case pressure of a 5.56 round can cause damage.
To this day, most published chamber pressure (MAP) data for cartridges is taken by the “copper crusher” method of measurement. The proper name for this type of measurement is CUP, or Copper Units of Pressure.
The .223 Remington is rated for a maximum of 50,000 CUP while the 5.56mm is rated for 60,000 CUP. That extra 10,000 CUP is likely sufficient to cause a failure in a chamber that’s only rated for the “sporting” .223 Remington.
The .223 Remington and the 5.56mm NATO, when checked with a chamber ream from a reliable manufacturer of each, also have discernable differences in the areas of freebore diameter, freebore length (leade) and angle of the throat. Exact dimensions for this chamber and many others can be found in this PDF available from ar15barrels.com. Notice that across all of the various chambers, there are differences in the exact measurements of bits and pieces. This can loosen and tighten the chambers in different ways for different reasons, directly affecting both accuracy (through consistency) and reliability. Generally speaking, .223 Remington chambers are tighter than 5.56 NATO chambers.
NATO and SAAMI also measure the cartridges in different ways. While NATO tests barrels made for 5.56mm NATO chamber pressure at the case mouth, this is not the location used by SAAMI. The sensors NATO and SAAMI use to conduct the actual pressure measurements are different as well. These differences account for approximately 137.9 megapascals (20,000 psi) difference in pressure measurements. This means the NATO maximum service pressure of 430 megapascals (62,000 psi) for 5.56mm NATO is reduced by SAAMI to 379.21 megapascals (55,000 psi) for .223 Remington.
In contrast to SAAMI, the standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO. However, the differences in dimensions remain.
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