12 grams are made of steel. I am guessing they are over engineered because they lack burst discs, and you don’t want them exploding if say you left them in your car on a hot day. (CO2’s pressure changes with the temp)
The larger refillable tanks are aluminum. They have burst disks to prevent them from going over the pressure rating of the tank. Though you can circumvent this by putting in a steel disk. That’s a bad idea, though.
The really, really big tanks are steel though.
