I would think that ammo block would be heavier than 5 rounds of 5.56. I don't think I'd like humping those blocks around in 100 degree heat. It's an interesting invention, but not practical, IMHO.
Different barrels would mean differ point of impact. You could never have accuracy. I can see this as an 'entry' weapon for a 5 round burst-- oh wait-- they already have that-- its called a shotgun with buck shot.