Re: Retumbo in cold weather? My rifles suddenly started shooting Low.
There is no way to get a load that shoots the same in warm and cold weather. In my experience, the larger the case capacity and the smaller the bore, the more variation you will see as far as bullet drop changes in different temps. This will happen with any powder, yes some powders will be better then others at limiting velocity change in different temps but I would say your using one of the better powders already.
Only way to know is to shoot in these different conditions and make notes of it. The main problem is that most will develope a load in the summer time when the temps are warm and then shoot all summer, this is great and what you should do, only problem is that when hunting conditions show up with cooler temps, the rifle should really be tweaked to those conditions.
Best time for load development in my opinion are spring and fall when temps will be similiar to what you will be seeing in hunting season. This is of course for a big game rifle.
For long range varmint rifles, I recommend developing loads in the heat of the summer if that is when you will be using them to make sure they will be safe in all temps.
Simply put, welcome to the game, unless your seeing several feet of drop difference, you are not seeing anything unusual even with Retumbo.
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