Whats the shelf life on opened AND unopened powder in an air tight military ammo can??

IF the powder was properly made, it will last virtually forever in a sealed airtight canister stored in a cool, dry place. As long as the "ether smell" is there, and there are no signs of red dust flakes in the powder, and it pours freely w/o clumping up, it is OK.

As said, "properly stored," that is in conditions that you would live in comfortably, it should outlast you (unless you shoot it). Now if you leave it in a musty basement with the lid off... bets off.

Solvent smells, alcohol, either, acetone (like nail polish remover) are o.k. Keep the cap on so they don't escape.

Vinegar is the sign of deterioration. Beware. Happy New Year. Luck.