03-17-2020, 11:52 AM
Temperature sensitivity has to do with variations of muzzle velocity (MV) (what you can see) and the underlying variations in peak chamber pressure (which you can't see) as temperature goes up or down. There are a couple of publicly available tables that show values of how many ft/s per deg F, of MV you get from a powder for a higher/lower temperature. The PRS blog, IIRC, also did a couple of studies on Varget, H4350, and IMR4166 as to temp sensitivity of those.
CFE in the one table I have, is said to have a variation of 1.7 ft/s/degF, whereas Varget is in the 0.2's (H4350 also).
The variation, even in those studies, is not completely linear but mostly so. It tends to accelerate at higher temps and slows down a bit at lower temps.
The temperature of the cartridge/powder when you fire it is the main component, b/c at a higher powder/internal temp the powder can burn faster... chemical reaction and all.
Now, I did a small comparison of my own, of some RL-17 in my 6.5 CM, b/c I have some and it does nicely in my CM. RL-17 has a 1.42 ft/s/deg value in the table-- my comparison came up with 1.0. This was over a range 70-43 deg.
One other caveat, and this is my own theory: the amount of delta you see for temp change could be related not only to the powder but also to the case size and burn efficiency of the cartridge. What might be 1.7 in a 223 or 308 (whichever) might be more or less in a 6.5 Grendel. I suspect most studies were done with 308 ammo; I haven't seen any studies that showed how a particular MV change might vary in a different cartridge or in a more/less efficient cartridge.
So. For CFE my approach is, since most of my shooting is done in an indoor range my baseline is 70 deg... I don't normally go out into temps above say 85, nor do I do much sub-freezing shooting, so for most cases CFE does well for me. I probably wouldn't take it with me for >100 yd shots at 30 deg (hunting in Va in Nov-Dec)... but for general shooting - no problem as far as I'm concerned.
CFE in the one table I have, is said to have a variation of 1.7 ft/s/degF, whereas Varget is in the 0.2's (H4350 also).
The variation, even in those studies, is not completely linear but mostly so. It tends to accelerate at higher temps and slows down a bit at lower temps.
The temperature of the cartridge/powder when you fire it is the main component, b/c at a higher powder/internal temp the powder can burn faster... chemical reaction and all.
Now, I did a small comparison of my own, of some RL-17 in my 6.5 CM, b/c I have some and it does nicely in my CM. RL-17 has a 1.42 ft/s/deg value in the table-- my comparison came up with 1.0. This was over a range 70-43 deg.
One other caveat, and this is my own theory: the amount of delta you see for temp change could be related not only to the powder but also to the case size and burn efficiency of the cartridge. What might be 1.7 in a 223 or 308 (whichever) might be more or less in a 6.5 Grendel. I suspect most studies were done with 308 ammo; I haven't seen any studies that showed how a particular MV change might vary in a different cartridge or in a more/less efficient cartridge.
So. For CFE my approach is, since most of my shooting is done in an indoor range my baseline is 70 deg... I don't normally go out into temps above say 85, nor do I do much sub-freezing shooting, so for most cases CFE does well for me. I probably wouldn't take it with me for >100 yd shots at 30 deg (hunting in Va in Nov-Dec)... but for general shooting - no problem as far as I'm concerned.
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