Re: 500 yards and the weather?
Weather has lots of effects on ballistics but I would say that the more likely culprits would be the chamber temp when you fired. or if you have a wood stock that was greatly affected by high humidity and high temps. or simply flinching/ misreading the wind.
The terrain can affect longer shots quite a bit as well. Shooting over a valley will cause you to encounter much different wind patterns than shooting over a bean field. through clearings will obviously cause you to experience much different wind patterns as well.
The cause of shooting a much poorer group isn't caused by ballistics as much as it is caused by inconsistency from firing to firing.
Honestly, a 4" group at 500 is pretty darn good... it is pretty easy to misread wind at that range to get a bigger group --- or to flinch.
I used to re-load but now I "hand-load".
-- Well, at least I try