I agree with WildRose try the H1000. I have been using Retumbo in a 300RUM and 7mm Dakota this summer and just tried H1000 in those guns and the ES and groups both shrunk. I spent quite a few days at the range this summer when it was 112 - 118 and the H1000 was more stable than the Retumbo, in some powder charges the ES was very large with Retumbo in the heat.
Another thing is the heat you described should actually make the velocities higher so be sure to shoot when it is cooler. You did not mention if you saw any signs of pressure but if you haven't seen any signs I dont see why you cant test up to the max load, the pressures would actually be lower in the high heat. In the 300 RUM the difference from some of my range days to shooting in the mountains where it is cooler and there is elevation had a MV decrease as much as ~10%. Just some things to think about.
Thanks for the input. So far I have tried H-4831, RE-22, H-1000 & Retumbo and haven't been able to find a load that beats the first 3 shots fired out of the gun. While at the gun range yesterday, I tried 63.0 G of Retumbo and shot 3-shot groups with the bullet depth anywhere from being jammed into the lands to .050 off the lands. The best 3 shot group I got 1/2". I got very good velocity out of this load but it left me scratching my head. In almost all of my tests, shot 2 was 1 1/2" from shot number 1 but then shot #3 cut the hole of shot #1.
The good news is that the weather has significantly cooled here in Phoenix, and I'm on vacation next week so I plan on hitting the range hard. I'll try to post some more pictures as I seek advice.
I just built a similar rifle, if your barrel is 1-9, we have the EXACT same barrel.
I pretty much only use 4831sc because of similiar temps here in the Central Valley of CA. I stopped at 63 g with nosler brass and Fed 210m primers, 140 ttsx bullets, with no signs of pressure at 92 degrees and 3090 fps. I laddered 59.5 to 62.5 from 300 yards and they all came in touching (in steel) with the exception of the 60 g load jumped high about 2 inches.
In my particular rifle it seemed the more powder you put in it the smaller the groups got. The day I laddered I had a pre-determined stopping point of 63 grains (if I got there safely) and had 3 rounds loaded. After the ladder I backed up to 400 yards and in some mild mirage shot a three shot group that went about 1/4 moa vertical and a little over half moa wide, and just called it good. load is 50 off lands and 13 fps spread over three shots.
Side note: Rifle has shot 150 sst, 150 smk, and 140 ttsx under half moa at 200 yards with 59 grains.
~Jeffwhip - your rifle is saaaa-weet!!!! WOW - what a nice shootin' iron! ALso, i like the reference folders on your shelf - OT: Prophets, Gospels, etc. Reminds of Seminary and trying to keep 'relevant' notes on areas of Scripture.
Now, don't cause yer brother to stumble w/ the nice LR rig ya got there!!! Go 'ugly it up' w/ some hunting scars!!!!