Correct Tony, & that is why they are NOT interchangable (Except in a 'take your own chance on it, in a Battlefield scenareo) the usage of a US Round in the SA system would damage the internal walls of the barrel & VERY quickly wear it out! On a hair splitting term: Yes it would fire & opperate. But NO, it is not permitted & would damage the barrel.
So much for "NATO standard", what was the point! it is interesting to see the private initiatives from manufacturers to produce larger calibers ie 6mm and 6.8mm to meet shortfalls found with 5.56mm.
I watch too much TV!