Just ran a fairly rigorous temperature sensitivity test on IMR 8133 and Reloder 26 in my 7mm RM.
Equipment.
26" X-Caliber prefit, 1:8 twist, fluted heavy sporter.
Sartorius high precision scale for powder charges ( ≤ 0.03 gr ES for charge weights)
LabRadar
Indoor/Outdoor thermometer set (crappy, but within a couple degrees of 2 other crappy thermometers)
175 ELD-X, RL26 and IMR8133, Fed 215M primers, R-P brass, 3.335 OAL (~0.020 off lands).
Method.
1) All rounds were loaded in one session, using my standard methods. Neck turned, ~4X fired, annealed every firing, FL sized (shoulder bump 0.002"), sinclair neck mandrel expanded, and trimmed. Pin tumbled, graphite lube in neck.
2) An aliquot of 5 rounds was placed in a cooler with ice (~32° F) for transport, and a second aliquot was placed in another cooler with 3 Nalgene bottles full of boiling water. Since it was a steady 29º F for the shooting session, the "Cold" aliquot was taken out of the cooler, and left outside on the ground next to my shooting position, with a thermometer in the lid of the ammo box. Both aliquots had ample time to come to a near equilibrium temperature (>2 hours), though the "Hot" rounds obviously slowly cooled during transport and setup from ~135° F initial temp, to ~100° F when fired.
3) Rounds were fired in 4 shot strings, 2 "Cold", then 2 "Hot" (to ensure maximum chamber heat for the "Hot" aliquot). The barrel was then allowed to cool to ambient temp before the next 2x"Cold" and 2x"Hot" string of 4. The last two rounds were fired 1x"Cold" and 1x"Hot", for a total of 5 shots of each temperature.
4) Both sets of loads were on the toasty side for this rifle (I was exploring top end loads for these powders), and based on the velocities, are probably at or slightly above SAAMI max.
Results.
Reloder 26 - "Cold" average = 3010 fps, "Hot" average = 3015 fps. ~0.1 fps/deg
View attachment 237216
IMR 8133 - "Cold" average = 3060 fps, "Hot" average = 2989 fps, -1 fps/deg (dubya tee eff!?!?!)
View attachment 237217
Discussion.
Reloder 26 performed admirably, with an ES of 21 fps and SD of 8 fps across all shots.
IMR 8133...I don't know W.T.F. is going on here, and can think of no explanation. ES/SD was rather high for each aliquot, but well outside error. I've replayed the shooting in my head over and over, and I'm 99.9% certain there's no way I reversed the shot order, as I intentionally shot the "easy to grab" cold rounds out of the box at my elbow first, then shot the ones I had to dig out from under a wool shirt and inside the cooler, 2nd. I know for a fact that the last two were 1x"Cold" then 1x"Hot", as I pulled some hot water out to give to my dogs since the experiment was finished, and those two also showed the extreme difference (Cold 3078 fps, Hot 3000 fps). As also mentioned, I loaded these in the same sitting, and they were only separated into aliquots this morning when I put them in their respective coolers.
The results for IMR8133 are so odd, that I think it requires a retest before putting much stock in the outcome. I will hopefully get a chance in a week or two when the grubby weather that's inbound has passed. I'll probably use my 264WM for that, to get another data point.
