From the article linked in post 15:
Initial SOF requirements included two basic versions of SCAR system - the SCAR Light (SCAR-L), available in 5.56mm NATO, and the SCAR heavy (SCAR-H), which should be initially available in significantly more powerful 7.62x51 NATO chambering, and should be easily adaptable in the field to other chamberings. These other chamberings initially include the well-spread 7.62x39 M43 ammunition of the Soviet / Russian origins, and probably some others (like the proposed 6.8x43 Remington SPC cartridge, especially developed for US Special Forces). The key idea of SCAR rifle system is that it will provide the Special Forces operators with wide variety of options, from short-barreled 5.56mm SCAR-L CQC variation, tailored for urban close combat, and up to long range 7.62x51 SCAR-H Sniper variant, as well as 7.62x39 SCAR-H, which will accept "battlefield pickup" AK-47/AKM magazines with 7.62 M43 ammunition, available during the operations behind the enemy lines.
The 7.62 x 39mm is an East block round. It is used in the AK-47 that just about all of the people we're opposing have at the moment. Would be nice to pick up and use their own ammo against them, eh?
I guess, but will the troops have to qualify on each round? Each will have different firing characteristics and a different point of impact at different ranges. If you wanted to simply spray and pray in a firefight why not just pick up a dropped enemy weapon or pick it up off a dead enemy and shoot? Seems unnecessary to me.
Now, I can see having the capability to shift between 5.56 and 7.62 x 51 with a barrel change to go along. That would shift the effort from an assault rifle in the 5.56 to a possible designated marksman in the heavier load and longer barrel. That makes up for the middle ground between the rifleman and the sniper.
The rifleman covers the area from the muzzle out to about 300 meters. The sniper engages at 700 to 1000+ meters. The new category of "designated marksman" (born of combat necessity) covers the ranges from between 300 and 700 meters. That is the perfect operating range for the .308 Winchester with only minimal training.