The reason is NATO compatibility. No more, no less. That was the answer when I asked when serving. Now, having said this, there are .30 caliber individual weapo

The reason is NATO compatibility. No more, no less. That was the answer when I asked when serving. Now, having said this, there are .30 caliber individual weapons aplenty in troop hands and those that use them prefer them. They do perform far and away better. What most people don't understand is the troop loadout for combat ops. Men are not dripping with weapons and rounds per Hollywood. The standard loadout is an M4 rifle with 6 30-round magazines (180 rounds). If you are issued a handgun, that is 3 15-round magazines for an additional 45 rounds of ammunition, albeit in the "weak" 9mm, which is a get-off-me gun meant for house-to-house, close quarters fighting where a long gun would be unwieldy. As you suggest, there is a large push to go to the 6mm/243/6.5, which offers SUBSTANTIAL performance benefits over the 5.56. A .243 (6mm) is pushing more ballistic energy at 500M than a 5.56 does at 100M. That's a lot more. And the rounds don't weigh that much more or take up that much more space. Recoil, likewise, is negligible at best due to the gas-operated platform. But, NATO... NATO is sold on the idea of 5.56 and 9mm. I think that DMs should be issued the better long guns to deal with issues the 5.56 cannot handle.