Stoner 63A set up as a light machine gun
McGeorge Bundy's early-1960s choice of the Armalite AR-15 that was later developed into the M16 was the wrong path to follow. Eugene Stoner's M62/M63-A1 would have been a lot better selection with it's ability to change from a service rifle to a squad automatic weapon without the need for tools. The Stoner M62/M63-A1 was the rifle that Eugene Stoner was pushing, even though he was chief engineer on the Armalite AR-15.
If the US military had the refined Stoner today, we could choose different ammo selections for hard and soft targets before going into the field by switching out the barrels. The M4's problem seems to be it's shorty CAR-sized barrel that isn't getting SS109 projectiles to the velocities it needs to be lethal. The M4 doesn't appear to be any more lethal or reliable than the WWII-vintage M1 carbine, even though it's ballistics may look better on paper.
Maybe 6.5 Remington is the middle ground between 7.62 NATO and 5.56 NATO, but simply rebarrelling our inventory of M16-A2/M4s for the 6.5mm round isn't going to cut it. We'd still need a whole new rifle.
There's very few 5.56 NATO rifles out there that have quick-change barrels, but the best one out there comes from a company currently in financial limbo that could be bought for peanuts -- Steyr AUG. It's just my opinion, but I believe that the Steyr AUG is worlds better than every other NATO 5.56 rifle, even including the Swiss Stg. 90PE (SIG-550).
Australia has had great results with it in East Timor.
Stoner 63A set up as a light machine gun