No, the STANAG isn't optimal.
The 5.56 round is tapered, yet the AR has a straight magwell. Original 20 rounders were straight, but not long enough to encounter enough ammo-stack curvature to make that setup not work. They angled the floorplate to compensate.
30 rounders have a long enough ammo stack that the mag has to curve. It's a fact of geometry. So the best that could be managed with a straight magwell was the STANAG's solution - straight neck with an angle just below it. So you're trying to put a smoothly curving ammo stack into two straight boxes with a joint in the middle. Not the perfect solution.
The Pmag is an improvement, since it has a curved lower section after the straight upper neck. It still has to work around the straight AR magwell, though, so it can never be perfect.
Better solutions are similar to the G36 and AUG approach, with a continuously curved mag that isn't constricted by having to feed a curved stack of tapered cartridges through a straight magwell. 5.56 AK mags, despite the bad rock-and-lock ergonomics, also handle the geometry problem better. They are a continuous even curve, or at least very close.
But we're probably stuck with STANAG mags. It's a classic chicken-egg logistics problem - mags without the intrinsic STANAG flaw are desired to replace all the STANAG mags, but nobody wants to make them because there are so many STANAG mags already that people are using.
Until a weapons platform without the AR's stupid straight magwell replaces the AR in popularity and use (unlikely until our .mil changes rifles sometime this century), STANAG will remain the de facto standard.
www.ar15.com
