It's a light machinegun. The range/accuracy/control isn't going to change dramatically when you cut a few inches off of the barrel. The weapon is ineffective or only mildly effective well before the barrel's shortened length comes into play.
The only valid point there would be the "damage" being higher for a longer barrel at closer distances, but even then we're talking about maybe 20-40 yards of extra increased effectiveness. Maybe 50-75 if we're being extremely generous, and even then, unlike a rifle where you're only hitting the guy with a shot or so every time, an extremely easily controlled LMG like the 249 or Para is going to put a lot of rounds on target at your average engagement distance.
So, in short, you're exaggerating the difference between the two weapons. Congrats for knowing that the barrel is shorter and that M855 fragments at "high" velocities (above 2700fps, to be precise), but to jump to your other conclusions based on that is erroneous.