Why the short barrel? As I recall, the cartridge was develop with a shorter barrel in mind. The 25mm payload caliber is able to be shot from the XM-109 nearly as well as the .50 BMG. It is also considered a light weapon. So, a longer barrel would not have all that much to give unless the shooter wants to shoot beyond 2000 yards. Most foot soldier fighting is well inside that range.
Is the short barrel to reduce wait? Maybe, sort of. No doubt it will be lighter with a shorter barrel, but the gun is still heavier than the larger 82A1.