SSK, the company that created the .300 whisper and associated ammunition have developed a 6.5mm cartridge base of a 5.56 case, 6.5 MPC. From my limited understanding as of yet a new barrel is all that is required. The round is fully functional in M4 magazines.
I remember when the 6.8SPC round was announced and they said it would work in standard AR mags. After some "real" testing it showed that it would work for few rounds and then bulge.
Loading a 6 to 6.5 mm bullet into .223 cases has been done for years--it just doesn't offer enough. I'd much rather use a 6.5 to 7 mm projectile in a case with a bit larger powder volume.
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