CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
I agree with some of the information, but some of it is confused. When 7.62 NATO was developed, Winchester, who had helped with the development, immediately released the same cartridge with the civilian name, .308 Winchester. When 5.56 NATO was developed, Remington, who had helped with the development, immediately released the same cartridge with the civilian name, .223 Remington. At that point, there were no differences other than the name and the military requirement for brass hard enough to withstand full auto extraction, crimped primers, and sealed bullets and primers. Unlike 7.62/.308, where discrepancies in the capacity of military and civilian cases developed differences, 5.56 NATO has remained remarkably consistent. The only variation seemed to occur when Belgium developed SS109 (which we adopted as M855) and gave it what measured to be about 5% more pressure than our M193. This raised M193 pressure from 52,000 CUP to 55,000 CUP, and conformal transducer pressure from 55,000 PSI to 58,000 PSI, which is what Federal says it loads to today.
But here's the funny thing. Using Kisler pressure transducers, NATO measures the same round to be 4300 bar or 62,366 psi. Guess what pressure the CIP uses as standard for .223 Remington? Yep, 4300 bar or 62,366 psi. So if you buy .223 made in Europe somewhere, you are hitting the same pressure as in NATO ammo. The fact it's in a .223 Chamber mitigates it a bit, but not dramatically.
So how could this happen? Well, it's a matter of limited absolute precision in pressure measuring gear. The bottom line is getting within 15% of absolute is about as close as it gets, and that explains part of the reason load data from different sources doesn't agree. I am unaware of any .223 made that won't tolerate any 5.56 load made. If someone actually has had sticky bolt lift or some other positive pressure sign running M855 in a gun with a .223 Rem chamber, I'd sure like to hear about it. The fact is the pressure difference, such as it is, and only on some measuring fear, is far less than the difference between a production load and a proof load. Far less than the 15-18% the CIP and SAAMI extreme variance allows for. It really just isn't an issue. The only possible exception would be with a load so long it hit the lands in a .223 chamber when it didn't do so in a 5.56 NATO chamber. It's a theoretical possibility for specialty military ammo. But not for the standard ball and AP rounds.