Right now there are two things currently being researched to give greater CQB abilities to troops. The military is working on a 6.5mm cartridge, and Knights Armament (who supplies the Mark 11 mod 0 Stoner SR-25's for the SEAL's as "Designated Marksman Rifles") has put out the Knights SR-47. It's an accurized AR-15/M4 variation with a free-floating barrel in 7.62x39mm that takes standard AK-47 magazines.

I support 7.62x39 over 6.5mm because I personally know people who have testified how much penetration power you get from standard (cheap as hell) Chinese factory ammo. It's not the highest quality, but you can also get it with an air pocket in the nose much like the 5.45x39mm Russia has: which means not only will it penetrate just about anything but if it hits any soft tissue it's going to swoop through a body. As the Afghanis called it, "poison bullets."

Also: from an M16A2 (with a 20 inch barrel) the 5.56x45mm rounds are fired at high enough velocity that when it hits soft tissue the bullet splits apart and the lead splinters in every direction leaving not temporary but permanent wound cavities. The problem is that the M4 and it's other 14.5" barreled cousins don't fire the 5.56x45mm at a high enough muzzle velocity for that to take place. That's why two rounds will generally drop a human from am M16A2 for 3 to even 4 are needed from the M4. It's not really the rifling twist that has an adverse affect. It's the muzzle velocity change caused by the extra 5.5 inches of barrel that the M16A2 has over the M4.

That's why a beefier, heavier hitting round is wanted. The new super small SMG sized 5.56mm weapons used by the USMC are promising. They defeat armor but the 5.56mm from a short barrel lacks lethality.

I think a 7.62x39 upgrade would be doable. The downside is the 7.62x39 has almost twice as much recoil as the 5.56mm, but a lot less than the 7.62x51mm that my uncle used to tell me about. (Or the .30-06 with which the U.S. won WW I, WW II, and Korea.)

It is also a heavier cartridge than the 5.56mm. Although you'd pack less ammo, you'd need less ammo. When one round generally kills (air bubble in tip and steel jacket), compared to the 4 rounds previously required to drop someone, you really need 1/2 the amount of ammo. (That's being conservative, I could have said 1/4.)

If the standard rifle was designed to take whatever 7.62x39mm round you could throw in it (like perhaps an SKS does), plus load standard AK-47 type magazines, than you can re-supply by raiding an enemy ammo dump. (This, of course, would be far more useful for guerillas than for a regular army.)

So I think Knights Armament Co. had a great idea, but I think the standard M4 built to load AK-47 mags and 7.62x39mm ammo could do with a few improvements.

1. An improved gas piston inside the upper receiver so that gas and debris no longer directly fouls the chamber. (Increase reliability and decrease cleaning.) I don't suggest the same style the AK used, but a gas tube that attached to a piston port inside the upper receiver that will push the bolt open and exhaust the last of the gasses out of the weapon rather than into the bolt.
2. Free float the barrel. You can clamp the front sight onto the forearm assist so long as the forearm and barrel are in direct correlation; then there will be no decrease in accuracy, and every soldier gets an accurate weapon.
3. Everything inside the chamber and the barrel itself should be chrome-lined stainless steel.

Throw these things together and you'll have a battle rifle every grunt will adore; highly accurate, much more reliable than previous AR-15 type weapon, and less maintenance required. (I'm thinking particularly about non-combat personnel here, whose weapons care is likely to be sketchy at best.) This would be ideal. Also exceedingly easy to re-supply and increased lethality/penetration, all the way out to 300 meters.

You could use 5.56mm (which is actually usable in an accurized rifle to 800 meters; an example is the Filipino Marines MSSR) in the standard M16A2 rifles as the main battle rifle, and 7.62x39mm in the 16" barrel carbines. That way you've got optimal firepower long and close range. Completely do away with the MP5-N for CQC and just use the new M4 7.62x39. And modify the M16A2 with the same modification I suggested for the 7.62x39mm carbine.

Well, that concludes my rant. I wrote it because I don't like the direction that the military is going with 6.5mm. Support Our Troops!