Sig MPX in 300 BLK -- is this the PDW that we have been seeking?

[Klem]

Why wouldn't .300 BLK (fired from a 5.5" barrel) be effective out to 200 meters?

I'm thinking a 110gn TAC-X would be doing about 1200fps by the time it gets to 200M (MV = 1,700fps from a 5.5" barrel), and a 24" drop from a 50M zero. Pistol type performance requiring some skill to cope with the rainbow trajectory. 200M is a big ask for such a short barrel.