I was just watching videos about guns on the YouTube and saw how the bolt is pushed back once a bullet is fired. so i just wanted to relate the bolt of a gun with the piston of a car.
Well it is possible to use gunpowder as a fuel in automobile but it has never been considered as ideal fuel. No one has made successful gunpowder engine till now. Gunpowder is a solid fuel that does not burn clearly and build residue which is harmful for engine. The concept of using gunpowder as a fuel was firstly explored by Christiaan Huygens. However they never get succeeded in justifying it as an ideal fuel that’s why diesel and gasoline engines are still in trend as you can see at http://www.buyusedengine.com/used_engine.php.