MAGCARGO's shell is actually its skin that hardened as a result of cooling. Its shell is very brittle and fragile - just touching it causes it to crumble apart. This POKéMON returns to its original size by dipping itself in magma.
Sapphire:
MAGCARGO's body temperature is approximately 18,000 degrees F. Water is vaporized on contact. If this POKéMON is caught in the rain, the raindrops instantly turn into steam, cloaking the area in a thick fog.
Fire Red:
Its brittle shell occasionally spouts intense flames that circulate throughout its body.
Leaf Green:
The shell on its back is just skin that has cooled and hardened. It breaks easily with a slight touch.