If you really want to overkill then you should either use vacuum sealing or drop your desiccant in and put a bit of liquid nitrogen in the ammo can. Lay the top down (but don't latch it) and allow the expanding nitrogen "burp" out all the air before you seal the can. (Make SURE the nitrogen has all expanded or you'll blow up the can). Now you have a nitrogen (read: inert gas) filled, low humidity ammo can.
I suspect ammo stored this way will last in excess of a century, as long as the can is airtight.
I think this setup might even deal with extreme temperature changes (the worst problem for ammo). The problem with temp changes is that heating tends to evaporate moisture, allowing it to work its way into places it shouldn't be. Then any cooling will condense it and ruin powder, bring in contaminants, etc. etc.
Filling the ammo can with inert gas, solidifying all the moisture then allowing it to slowly warm and be absorbed by the desiccant leaves you with a massive overkill time capsule.
You can use dry ice too. Liquid nitrogen is, however... cooler. (Hee!)