The mythbusters already did the bullet in the oven thing. The cases have less mass than the bullet itself and requires less energy to move from a resting state and thus will travel much farther than the bullet. Neither had enough force to break the viewing window on an oven.
Now my guess is that a plastic ammo can would deteriorate before ammo starts going pop. That being said i think your ok with the .50 cal cans. But a fire rated safe is always a better option.