The gunpowder will still ignite, but the moment the slug leaves the jacket, all of the energy is released and little of it propels the bullet. If the round is in a gun, it is propelled with the full force of the chemical expansion, and all in one specific direction down the length of the barrel.
Compare the water calmly burbling out of your garden hose before and after you stick your thumb on the opening, either dispersing the force over a large area (the opening of the hose) or channelling it all in one direction.