I have to agree with foxx, some powders are more sensitive to temperature change than others, also how the rifling is cut. A good example is a friend's 300WM loves IMR 4064, while my 300WM loves IMR4350 with everything else being equal. When I use 4064, my groups expand to more than double, but when he uses 4350, his rifle shoots equal to mine with the same charge. Go figure......