They chose the HK IAR because it flat out outperformed the other entrants in every category, met or exceeded every standard and kept it lightweight. Think about it this way, with the urban combat the troop are seeing today the M249 can be a hassle in closer quarters. the lighter, 16.5" barreled M27 will be easier to carry and still be able to provide sustained fire. there will be higher volume magazines for it later, for instance Magpul just put in a patent for a quad stack magazine, other manufacturers should soon follow to try to get the contract. I think of this as a good thing, it seems the brass is looking out for it's men by giving them something that is proven to work (the 416 has been fielded for years) as opposed to army trials where they stuck with Colt's M4, the poorest performing weapon of the bunch in their testing, making it seem like someone up top had a hand in Colt's pocket after the large order was placed.
edit: barrels were not feet long LOL