Re: ballistic coefficient questions . 6.5 mm bullets
The .491 is a logical average falling from 3kfps MV, probably under ICAO conditions(SL, 29.92"Hg, 59deg).
Now add std conditions at 6000ft(38deg, 24"Hg), the same bullet goes to .587 and another 250fps beginning adds ~.007, so you're looking at .594 for a local BC.
Be sure to test it