jski asked:
How vulnerable are smokeless powders to humidity?
Smokeless powders are very stable. Humidity and high temperature will accelerate their decomposition and so are best avoided or minimized.
Should they stored in a climate controlled environment?
That would be desirable, but not necessarily mandatory.
Because of being sidelined for a while due to a neurological condition, I have partially used containers of:
- Bullseye
- Red Dot
- Green Dot
- Blue Dot
- Hercules 2400
- IMR-4198
- IMR-4227
- DuPont 680
- Hi-Skor 700X
that were purchased between 1978 and 1983 that were stored in a garage in northern Arkansas on a catfish farm. Summer temperatures approaching 100 degrees and relative humidity between 70 and 80%. In 1992, that powder was moved to a garage in a suburb of Dallas.
Beginning in 2014/2015, I checked the powder. There was no rust color in it and the smell was still right. I started reloading cartridges with it. I then compared cartridges loaded in 2014/2015 with cartridges loaded with the same powder prior to 1993.
The DuPont 680 could not be tested as I no longer had an M1 Carbine. But amongst the others, almost all 2014/2015 cartridges returned chronograph results that were essentially identical to what had been loaded two decades or more earlier and were consistent with velocities recorded in the 1990s. The exceptions were Green Dot which returned velocities 40% lower than the loaded cartridges and IMR-4198 which returned velocities that were - after accounting for the statistical "noise" - about 5% lower.
So, in at least one situation, powders stored under less than ideal conditions for as long as 37 years generally looked, smelled and performed the same as they had decades earlier. So, are high temperature and high humidity bad for powder, yes. How bad? My experience is that they will accelerate the rate of deterioration, but not by a whole lot.