How are 'self-sealing gas tanks' made and how do they 'self-seal' when punctured by a bullet?

Not sure of the manufacturing process but the self-sealing fuel tank was a normal fuel tank on the outside. On the inside was a layer of rubber or plastic-type product that melted due to the kinetic energy of a bullet piercing it. The melted rubber would fill the hole created by the bullet.

To add slightly to the above, the fuel tanks were covered on the OUTSIDE with a layer of raw, non-vulcanised rubber. When a bullet passed through, the rubber closed behind it slightly, and a little fuel often leaked out, causing a reaction with the rubber, making it swell and plug the hole.

Phil