There’s some pit picking occurring, to the point that folks might be missing forest for the trees.
M193 was the end result of years of development for a round for use in the AR-15, eventually adopted in US service as the M16 and M16A1.
Eugene Stoner originally designed the AR-15 around the .222 Rem, but when it became obvious it lacked the powder capacity to meet the expected performance of the US military he developed the longer. ”.22 Special”, which eventually became the .223 Rem, in conjunction with Remington and Sierra. The original bullet, as designed by Sierra and later called the “type B“ bullet had a long 7 caliber ogive and a high BC that enabled the round to retain the velocity needed for the 500 yard 10 gauge steel penetration test in the .22 Special/.223 Rem at 55,000 psi with readily available powders.
Remington however used its own shorter 5 caliber ogive, lower BC, “Type A” bullet when developing the M193 round for mass production. The shorter bullet helped address some cold weather marginal stability issues in the 1-14” twist barrels used originally. However, 1-12” twist was later adopted anyway, so it was a wasted effort caused by an initial reluctance by the US Army ordinance folks to adopt 1-12” twist.
Unfortunately the shorter, lower BC bullet lost a lot more velocity with the result it could not meet the 500 yard penetration test. That lead to a series of waivers for increases in maximum pressure and a search for a powder that could achieve the required muzzle velocity within even the higher pressure standards. And ultimately, the required penetration range was shorted by 100 yards anyway. Remington was not willing to use the Sierra bullet and the ordinance folks apparently couldn’t understand why it made a difference.
Remington also introduced its .222 Magnum round and Stoner later said if he’d known Remington was developing it, he’d have just used it instead of developing the .22 Special.
The end result by 1967 was the M193 being adopted and standardized with the shorter Remington bullet with a 58,000 psi maximum average pressure limit, up from the original 55,000 psi limit for the .223 Rem, and a resulting increase in free bore diameter and leade length in the 5.56x45 chamber to address pressure concerns.
Fast forward to the mid 1970s and there was a move to adopt the 5.46x45 as a substitute standard NATO round. NATO however wanted a longer 700m penetration distance and developed the 62 gr SS109 projectile incorporating a steel penetrator. It was ultimately adopted as the NATO standard (not just the substitute standard) in the form of the SS109 NATO (and US M855) round, with a maximum average pressure of 420 MPa (60,900 psi).
It was ironic as the USMC and US Army had been developing a more accurate version of the M16A1 using a heavier barrel and round hand guards. But it was eventually adopted in modified form as the M16A2, with a 1-7” twist to accommodate the very long M856 tracer round, and firing the M855 round with the not very accurate SS109 projectile. Within a decade it was in second line use as troops who could, ditched the now overly long, over weight, and not suitable for use with body armor A2 for the M4.
And of course the M855 round in the short barrel M4 lacked both the soft body effectiveness of the M193 and the long range penetration intended by NATO for the SS109, leading to the present perceived need to develop a new 6.8mm intermediate round.
So, development from the .22 Special to the M193 and the SS109 adopted by NATO
was a fine example of what happens when things get designed by committee with members with competing, and often changing goals and interests.
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Technically then .223 Rem, 5.56x45 M193, and 5.56x45 SS109 NATO are three different cartridges with three different maximum average pressure standards and two different chamber specifications.
The NATO cross however seems to be mis understood. A NATO cross head stamp just means the ammunition is compatible with the NATO standard and is safe to fire in NATO weapons.
Thus you will find M193 ammunition that is indeed compatible with NATO 5.56x45 firearms even though it is not the “NATO standardized“ SS109 round. That was in fact the case from 1980 when NATO adopted the SS109 until late 1983 when the US military finally adopted the M16A2.
You‘ll also find numerous examples of 7.62 NATO ammunition with the NATO cross that isn’t the NATO “standard” either. You’ll find 7.62 NATO rounds with projectiles weighing 143 to 155 gr that are more or less M80 equivalent rounds, all with the NATO cross denoting compatibility with 7.62 NATO chambered weapons.
