BTM,
I am assuming that your H-S stock is free floated, so I would eliminate that. If it is not free floated, get it free floated. It could be the factory barrel does indeed have stress in it, due to not being properly stess releived from the factory. It could also be due to parallex, but with a 2.5-8x scope, you can' t do much about it.

There again, it very well may be changes in the weather conditions. Changes in air temps, barometric pressures, humidity, winds, etc., always have some kind of effect on a bullet' s trajectory and flight, some times minor, some times major effects. As the ranges increase so do the amount of the effect of the atmospheric conditions.