That defeats the whole purpose of the intermediate cartridge: to give the infantryman more firepower and more mobility. A 6.5 x 51mm cartridge would probably require a nine pound weapon to avoid soldier fatigue in combat.
What people are telling you about using sniper or crew-served weapons beyond 600m is straight doctrine. Remember, a combat unit is not individual riflemen on the range. The power of the unit resides principally in its MGs and mortars. The riflemen keep these vital power-projectors from being flanked. There is a range at which your prime weapons are your crew served weapons; and there are special purpose weapons available for the rare situation where you need to bring aimed fire on individuals at 600+ meters. You should see what happens when you put scopes on the SAWs and M240s (which we have done). It's a beautiful thing, unless you're an enemy.
You can suppress at 600-800m with the M4/M16 as it is. The accuracy of the weapon is seriously degraded at 800m but it is definitely accurate enough to get the enemy to put his head down. But with a 6.5x51mm weapon, like a 7.62x51mm one, you can't carry enough ammo to go around wildly suppressing stuff.
Afghan or Iraq are a bit rare in that you do get long range shots. In Vietnam a typical sniper shot was 150m. Most firefights took place at under 20m! In Europe in training, I very seldom could see 100m straight, let alone have hopes of engaging a target there. Infantrymen are not meant to go fighting in the wide open spaces -- that's why those dudes in the rolling foxholes still have a place in the Army today. (It's also why we like to have an Apache overhead when we are out in the open like that).
Many people focus on the rifle as if it existed by itself, in a vacuum. No, it is part of the system that is a soldier. In turn, he is part of the system that is the unit. While I don't doubt that, say, the .270 Win would be better for shooting THAT guy THERE dead RIGHT NOW, it would be a grievously bad decision for the system that is an SF team, an infantry company, or an entire Army. (Weight of ammunition not only impacts your soldier but your whole logistics train. You might wind up needing double the air sorties to transport ammo. Is it worth it? I dunno -- that's why we send smart guys to general staff colleges, to answer questions like that. But it has to be considered by people who can make informed comparisons, which doesn't include me).
d.o.l.
Criminal Number 18F
Question: What would be the best rifle for the "Second Amendment Miltia", which would not have those assets available to it?