Re: Temp sensitive powder and fps ?
Do you chronograph your loads?If not you will be chasing your results 'till Kingdom Come. From your post I gather you are estimating your MV.
All recent production IMR powder is "Extreme" Technology. Doubt temp is an issue. Back when the NRA tested this they used 4895, 4350, and 4831 -surplus. The 4831-surplus was bad. But the 4895 wasn't too extreme, about 80fps as I recall. But this is not your problem.
You have to have an accurate (<1% preferred) Muzzle Velocity to start. So buy, beg, borrow a chronograph.
But from your results I would guess your BC figure is wrong, that it is not what the bullet maker claims (surprise, surprise!). I use Sierra GK and MK for most of my shooting and their BC's are very accurate. That is not true for every maker. Because High BC bullets sell better the #'s are inflated. When you side by side the bullets and two are pretty much identical then how is it they vary so much in BC?
Many posts to this website have commented on reducing the BC # to conform the data to the prediction of the Ballistic Programs. I bought a second chronograph to get down range velocity (on the same shot, each shot) to get an accurate BC. I believe this is your problem.
Hope others will chime in on this. Dialing in your data is what it is about. Lots of experience on this site doing just that.