Imagine two experiments A and B. In experiment A I load a gun with a cartridge and shoot a bullet. Total energy in this experiment will be E(heat) + E(projectile). I include recoil in E(projectile). E(heat) in its turn will heat the muzzle and some air. Done. In experiment B I do everything the same, except the is no bullet (e.g. no projectile). Same gun, same cartridge and the amount of gunpowder in it. So I pull the trigger. Total energy of this system will be E(heat) which will amount to the same level as in experiment A since I am burning the same amount of gunpowder under the same conditions. I cant quite wrap my head around it.It turns out that in my experiment A I got E(projectile) out of nothing? Since it obviously didn't convert from E(heat) as clearly demonstrated by the experiment B. Someone please enlighten me !