Ever hear of folks that buy surplus ammo? Or folks that find great-granddaddy's WWII bringback? Ammo gets stored in not-climate-controlled settings all the time, and unless it has been submerged it goes bang without a problem.
Storing rice in your ammo might impart a metallic taste to the rice. Depending on just how bad the situation is you might find the taste unacceptable.
Relax and go back to worrying about the Earth instantly freezing as Venus (or is it Mercury) transits across the face of the sun tomorrow.