Re: 6.5x284 drop chart
Your velocity seems to be in the right range for your load and within 15FPS of what I get with my 24" barrel. Did you re-confirm your 200 yard zero when you shot the 300 yard group? Also, is your scope height entry into shooter correct? It is unlikely that your .5MOA is effected by an incorrect velocity. Even it it were off by 50FPS it would be less than .25MOA of change in POI shooting at 300 yards with a 200 yard zero. Something else may be going on, zero, hold, rest, scope, etc. If everything checks out, I would use your actual drop data and enter it into Shooter.
I have the Berger reloading manual and found that their max loads using H4831sc is understated. Both my 6.5x284's and several other shooters I know have been able to use 53gr with no signs of pressure with the 140's seated at .1" out of the lands or less. Also, the 53 gr load should be used with LR primers, not magnum. This will get you a velocity increase to 2900-3000FPS depending on barrel length. I would still work up to this level in .5 gr increments to be on the safe side.
"Let us speak courteously, deal fairly, and keep ourselves armed and ready"-T. Roosevelt