As if our armed forces don't have enough on their plates choosing a new rifle. Here is the 6.5 MPC ( multi purpose cartridge ). This is a .223 case necked up for a 6.5mm bullet. Only change needed to run it in existing 5.56mm weapons is a new barrel. Four different bullets in the 6.5 MPC. Note all are seated to 5.56 OAL to run through M-16 magazines. Image courtesy of SSK Industries Original Defense Review article. The cartridge inventors page on this new cartridge. The great google provides.