" By 2000 yards, I'm guessing that the air resistance of the relatively blocky shape of the projectile had caused enough loss of spin to de-stabilize it somewhat."
Having some experience at long range artillery shooting, I would think that considering the type of sights that were being used would have as much if not more effect on the accuracy. At any rate the record would indicate that this gun was capable of hitting a gun with about 50% of its shots at this range. From a practical stand point seeing an opposing gun well enough to accurately sight on it at 2000 yards would be very hard to do, much different from aiming at a 20 x 40 white tarp.