I am fond of St. Marks ball powders.
My 308 and 223's LOVE Win 748, and I find them to be very stable in the heat. I developed my loads @ 60 degrees and they are very close to original above 90.
Win 760, not so much... Both my 243 and 25-06 aren't happy when the temp goes above the mid-80's.
So, has anyone had real experience with 780 in the heat? 760 isn't atrocious, but I am not a fan of having to down-load for the summer months.

I've had bad luck with Win 785 in hot weather. Never tried 780. I wish that a guy could look at a spreadsheet and see how sensitive that powder is to heat.

I've seen buddy's reasonable prairie dog loads developed in the winter go way hot in the summer. I try to shoot loads in the heat of summer if that is when I'll be using them. I keep notes with dates and temperatures of favorite loads. I am afraid that I write these notes in the reloading manuals as I won't as likely miss place those notes. Now I have some very old and ragged manuals around.

Some newer powders are getting better than the ones they replaced. Glad we are not stuck with cordite.

Do I recall correctly that you're considering 780, but don't have any yet? Send me some heavy 24-cals and I'll try to do some hot-weather testing for you in the 243. Comparisons to cool-to-cold, well, we'll have to think about that a little...

As an aside, I've read some VERY good things about the temp-resistance of the General Dynamics stuff being labeled by Alliant as 2000-MR. I don't know whether the entire PowerPro series does as well within their respective cartridge design parameters as 2000-MR does, but you might look that direction if you don't see what you want from the Win-labeled stuff.

__________________
"A government big enough to give you everything you want is a government big enough to take from you everything you have." --Gerald Ford

Am considering, but don't yet own; correct.
You should know by now I don't shoot heavy stuff, except in my 308.

If you have 780, I will send you some 24's & 25's....
But you can't just go plinking, I want load data from them

Well, if you're gonna put a bunch of conditions on it, maybe you can get your own 'data!'

I'll try a few lightweights in the 243 if you want, but I think 780 might be too slow for them. Don't bother sending 25s. The WSSM is enough more efficient than the 25-06 that 760 is as slow as I can go for lightweights. Interestingly, 760 appears to be noticeably more heat-resistant In the 25 WSSM than it is in some other cartridges.

__________________
"A government big enough to give you everything you want is a government big enough to take from you everything you have." --Gerald Ford

Thought I'd follow up with just a tiny touch of 780-in-the-heat data.

I made up some 780 loads for my 243 (Browning BLR, 20" bbl) a couple months ago under Hornady 100-grain BTSPs. It was an increasing-charge series which topped out near, but as I recall just below, book-max (I don't have the figures here).

The hot weather testing resulted in a pierced primer with the highest charge, and WAAAY too-high velocity. Definitely seriously over-pressure. Ammo temps for that test were ~125F, and rifle temps were ~140F.

It's been cooler here recently, so I fired the 'extra' cartridges I hadn't fired in the heat (highest charge and one charge lower). Ammo temps were ~75F, and the rifle was not above very roughly 90F. No pierced primers this time, and the highest charge slowed down ~50+ ft/sec.

Now, this testing is a lot different than what one sees on the 'net when people claim EPVAT-style testing. That testing, when I've seen any explanation/elaboration, only alters ammo temp, but not rifle temp. This test saw a difference of ~50F in BOTH the ammo AND the rifle, and it resulted in very roughly a 1 foot/sec/degree change.

Whether that's good or poor temp-resistance, I guess I can't say. I'll not likely get a chance to do cold-weather testing of this powder, as I have no plans to buy more; I don't expect to shoot the heavies much in this rifle, and 780 is too slow for varmint-weight bullets.

__________________
"A government big enough to give you everything you want is a government big enough to take from you everything you have." --Gerald Ford