Here is a good explanation from a friend:
"When the U.S. adopted the M193, in its initially developed form it was in a SAAMI-like chamber and measured a maximum average peak pressure of 52,000 CUP at the chamber, which is how SAAMI came to adopt that number for 223 Rem originally. Then over the next couple of years, the military chamber was refined to add some freebore and taper the back end a little more and the same loads then shot to 50,000 CUP and that became the military's maximum pressure for that round. This is handy information because it tells you a 5.56 chamber will likely run about 4% lower pressure than the same round in a SAAMI chamber will. But that is just an average. Other factors can affect it. But I'll pretend the average is right for this post.
When SAAMI used 223 reference loads developed by copper crusher protocols to calibrate conformal transducers, which don't measure pressure through a hole in the case, but rather over top of the case, the readings they got indicated the transducers would read 55,000 psi at maximum. Thus, 52,000 CUP and 55,000 psi became the SAAMI MAP (Maximum Average (peak) Pressure) limits.
It was when the U.S. Military tried to get the M193 round adopted by NATO that the first real difference in pressure occurred. NATO felt the M193 didn't penetrate helmets at enough range to qualify, so they developed the SS109 round to use instead. This round reads 55,000 CUP on a military copper crusher or 110% of what the M193 does in the same test rig. The U.S. started to make the M855 round as our NATO compatible version of the SS109, loading it to that 10% higher copper crusher reading.
If you assume the same ratio of pressures going from copper crusher to SAAMI conformal transducer, you would expect M855 to measure 58,173 psi, and, indeed, some declassified information indicates that Federal and other ammunition contractors outside Lake City use a number very close to that to produce contract M855. The NATO EPVAT system, which uses a Kistler channel transducer sampling gas off a hole in the chamber just beyond the case mouth, measures these same cartridges as having a maximum average pressure of 4300 bar (62,366 psi), or a 7.2% higher magnitude than the conformal transducer calibrated with SAAMI reference loads does. Europe decided to adopt this pressure for the 223 Remington as well as for the NATO SS109. Based on the ratios, if they had copied the SAAMI copper crusher MAP, the channel transducer would be expected to read about 55,750 psi or about 3844 bar before rounding.
But then comes M855A1. This is the new Black Hills-developed "green" round being produced with a lead-free bullet. It is loaded to a still higher pressure using a faster powder to cut down on muzzle flash in short-barreled weapons. I don't have a pressure number for it at this point, but reports are that it erodes M4/M16 barrels faster. I would say that's probably finally getting to pressure I wouldn't want to use in a 223 Rem chambered rifle. It is this round that Black Hills can indisputably state is higher in pressure than 223. And if that higher pressure is multiplied by 1.04 to take the smaller 223 minimum chamber size into account, it could be a primer piercer and the source of pretty short case life.
Several things are apparent from the above. If I fire an M855 round in a 223 Chambered conformal transducer pressure and velocity gun, it is likely to produce a reading of about 58,200×1.04 or 60,500 psi after rounding. It's certainly a bigger number than the 55,000 psi that is currently SAAMI MAP, but it's still not a high number for a modern bolt-action rifle, especially not when you consider the little cartridge has thicker chamber steel around it and a smaller head area supplying bolt thrust than a 308 Win at 62,000 psi does. Additionally, the Europeans load 223 Remington to about 62,400 psi (after rounding) without having problems. The AR can digest it just fine, as it can actually digest the hotter M855A1, provided you are willing to accept faster throat errosion.
So, is there anything to concern yourself with about firing 5.56 NATO in a 223 Chamber? As long as the bullet doesn't jam the lands, I'd say no, with the single exception of the Black Hills M855A1.
So then we come to Western powder's list of 62,366 psi. I called them and asked them to confirm they were measuring it in a pressure transducer. Well, if that transducer is calibrated by copper crusher-compatible reference loads, then my expectation would be that they are producing a load that would read about 66,850 in a CIP channel transducer (note that the CIP uses the NATO pressure number, but uses channel transducers that sample gas through a hole drilled in the case instead of at the case mouth like NATO's EPVAT protocol does).
The bottom line, to my thinking, is to work up an accuracy load starting with SAAMI 223 data. If you want to head toward NATO pressure, multiplying the maximum load by about 1.017 will get you close to a load reflecting 58,200 psi by the conformal transducer with a number of commonly used powders. If you want to go still higher, take a look at Western's loads, but I suspect they are too high. They claim not and that they match the CIP readings, but I don't get that from the ratio of the charges which are more than 1.03 times their SAAMI pressure loads.
A final note: Realize that we are talking here about maximum average peak pressures for a sampling of ten rounds. It is not a required pressure, but an upper limit. To comply with NATO standards for both functional and ballistic compatibility, the military has to meet a minimum gas port pressure and a narrower velocity range than SAAMI calls for (to assure ballistic compatibility, meaning not only impact energy will be maintained but that standard sight graduations will be valid). What are the odds a charge will just happen to hit the gas port pressure and velocity numbers at the same time that it just exactly reaches maximum average pressure? Small. Many lots of powder will hit the velocity and gas port pressure numbers with a peak pressure well below that limit and do. Mix in individual gun variables, and I think this is why you see things like that test showing no difference in produced pressure. There could be a difference with some lots of some loads (especially M855AA1), but it isn't necessarily going to happen. Nobody forces the military or SAAMI to reach maximum pressure numbers and they usually don't. Those are just limits that keep too fast a powder from being used to reach the velocity spec."
