I don't have time to check stats right now, but it still shows up that the PGU-14 has a muzzle velocity of 1067m/s fired from the GAU-8, while the MK268 fired by the MK30 autocannon gets a hefty 1385m/s and is APFSDS, i.e. gets much less drag over its course. Same rounds, are you sure?
While I don't have figures for the US rounds yet (heh), I have racked up a figure of 57mm RAH @1000m/60° for the apparently similar German PMC-287 round. I find this figure hard to believe, but do you think Rheinmetall would publish official figures?
69mm @500m is largely believable for the GAU-8, and allows to kill
lots of of miscellaneous AFVs and some tanks in a totally improbable Stuka dive.