I guess my thinking that the NATO marking was to differentiate the caliber designation, not the loading, for .mil and .gov's who were likely to buy that rifle. Kinda like the stamp on the M16 that says 5.56. You can safely shoot all sorts of loads out of that rifle, not just M855. I have never seen it in print, or in a manual, where it is recommended to use lower pressure loads in an AR type subgun. A while ago I was doing lots of testing of various loads from my 16" carbine and then I tested the same with my 5.25" SBR. My research, as well as others', showed that velocity gains in the 16" were averaging 100-250 fps across all loadings of 9mm that I could come up with. This is all with factory 9mm loads, mind you, and the general understanding we came to is that the fast burning pistol powder used in pistol cartridges is used-up before the bullet leaves the barrel. Think about it, there isn't much of a fireball when you loose some rounds from that SMG. All the powder is burnt. Heck, history has shown that subguns like that love to eat heavy ammo like 147 and even 158 gr. bullets. Take the barrel off that Colt sometime and look at the construction of the chamber. It is built like an armored car. 9mm subguns can run much higher pressures than 9mm pistols. I think that 124gr. NATO loads are a little anemic from a subgun.