I had always heard that IMR7828 was Winchester's choice for the .264 Mangleum. It's one of the VERY few times a canister powder was used for a factory load.
On so-called temperature insensitive powders, there are so many "yes but" and "only if" and "except when" disclaimers attached to them that I treat such advertising claims as essentially meaningless.
I pulled a factory 140 power point a few years ago, it looks like they switched to some sort of ball powder that looked a lot like 780
I know they're using canister 780 in at least one factory load these days.
I pulled a Winchester Supreme .270 Win 140gr Accubond (because they shot so dang good in my rifle). There was a ball powder in it that weighed 57.5gr. Hodgdon data listed 58gr as max with a 140gr Swift, so I bought a pound of 780 and worked up a load with Accubonds. Sure enough, when I got to 57.5gr, it clocked the exact same speed as the Winchester Supreme factory loads, so it appears to be canister powder in that load at least.
