The actual round that paralleled the development of the M249 was called 6mm SAW, it looked more like a 5.45x39mm round. The slug weighed 105gns and run at about 2500fps,....XM732 Ball. The 6x45 is what they ended up with but it started out as a 6x51(243Win.?) and they even experimented with basically a 17 Remington too(4.32x45),.... can you imagine the problems there.........

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