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Another 6.5mm (.256) option is the
6.5 MPC which is discused at The High Road (THR) forum (which also weights 6.5 Grendell vs the 6.5 MPC vs the 6.8 SPC). The 6.5 MPC also has a detailed article at Defense Review on the 6.5 MPC

So, why the 6.5 MPC instead of the 6.8 SPC? Ease and cost of conversion (weapons conversion), ammo capacity, and ammo weight (ammo carry capacity at a given load weight). The 6.5 MPC utilizes standard AR-15/M16/M4/M4A1 magazines and bolts, and will function in both the SOPMOD M4/M4A1 Carbine and belt-fed FN M249 SAW/LMG, provided you switch out the barrel(s). No further modification is reportedly necessary. Mag capacity for the 6.5mm MPC is 30 rounds (although you might still want to down-load it to 28, as many do with 5.56mm ammo for reliability purposes). The 6.8mm SPC doesn't stack properly in standard 5.56mm M4/M4A1 mags, and the magazines that have been developed for it limit ammo capacity to 25 rounds, as opposed to 30 rounds, so the 6.8 SPC mags will fit inside current military mag carry pouches. You can also use 5.56 NATO stripper clips to load 6.5 MPC rounds into the mag. At present, there are no 6.8 SPC stripper clips.

The 6.8mm SPC doesn't stack properly in standard 5.56mm M4/M4A1 mags, and the magazines that have been developed for it limit ammo capacity to 25 rounds, as opposed to 30 rounds, so the 6.8 SPC mags will fit inside current military mag carry pouches. You can also use 5.56 NATO stripper clips to load 6.5 MPC rounds into the mag. At present, there are no 6.8 SPC stripper clips.

The Australians seem to be VERY interested in the 6,8 SPC and a demonstration and test fire was arranged for them at Camp Robinson, Arkansas back in 2004. Since their F88 AusSteyr variant of the Steyr AUG has a quick-change barrel, all they need is a quick field strip of their weapon to replace the bolt and barrel, change to the new 6.8 magazine, and they're good to go. Of more concern to them was [at that time] the lack of a usable 6.8 caliber link for the 5.56mm M249 SAW, known in Australian service as the F89.

If there's anywhere that the better ballistics of a 6.5/6.8/.280 upgrade version of the 5.56 cartridge would prove itself, it's in the longer ranges in the squad support weapon role. And with the first issue of the new weapon/ammo to be linked, the older linked ball ammo could be relegated to rifle training.