While I havent experienced any 900 yard kills on either end it seems to me that it would just have to be a luck shot, according to Shaws book/gunnery chapter you would have to have your nose at least 16 ft above the targert to overcome the gravity accelleration drop, in addtion the energy loss at range would be tremendous as would projectile sepatation, and with your nose that high and leading to boot I'd be surprised if you could even see the target
So the above 900 yard test leaves me wondering where the nose up attitude is or should be to get the kill? I know AH models the gravity drop as you can see the arc of the tracer fire.
Is Shaw wrong that with a muzzel velocity of 2880 you should see a 16ft drop in at one second out? Even at 300 yards and on 6 you should be aming about 5 ft above you target so 900 seems like a miracle shot to me and one that should seldomly occur.
Not sure what the weight is of the 20mm round in question but imparted energy should drop off much faster that mg's so it seems like pyhsics says you got damn lucky