Here's a great trick for preserving ANY powders or pellets.
You'll need a "FOOD SAVER" or other vacuum packing machine, but just put your powder or pellets in a roomy sleeve, and suck the air out and seal her up. No way any ambient moisture can get in.

Food Saver machines are awesome for storing more than just foods or meat for a loooooong time. Just found a packet of Oryx backstraps in the back of freezer from 2009. No freezer burn & still delicious!
Also a half used box of Pyrodex pellets from 2003. Took em out & fired em all. They worked perfectly... just like the Oryx steaks.