Heat, probably not. Humidity, I'd guess yes, since there's not really any air in there, but what little air was in there would still have the humidity percentage of the original air. Probably be better off with a sealed dry pack 'blue bag' inside of a sealed ammo can. On the other hand, I have no idea what affect negative pressure has on powder and primers!
I could get back to you in 10 years, but the red hats will probably have rounded me up and dirt napped me long before then. I just wanted to see how well it would seal with ammo.
It did a sterling job on the 5.56. The .45 has bigger voids to suction, but it did a pretty good job.
Now the company line would be:
- 9 out of 10 dentists agree!
- Our top scientists in the field say "yes"!
- Choosy moms choose Jif.