I think and I must emphasize THINK that it's 5.56 NATO that's the issue .
( Im not sure on the numbers but im going to put some out there just for example )
You have 20 different countries with who know's how many manufacturers in each of those countries making the same product .I would expect that not all the ammo would be manufactured to the exact same tolerances . My guess is the 5.56 chamber has a longer lead/throat to make sure no mater what country your getting your ammo from it will shoot safely and reliablely .
As of this date 8-18-14 at 6:42am I became a proud grandfather I guess I'm officially old