Warning.....Buzz kill.......I'm afraid your balloon would have to be made of asbestos or the muzzle flash would burn a hole through it when the gun fired. But, just for grins, let's suppose you have a balloon made of magic material that won't have a hole blown in it. It will inflate with hot gasses. the size of the resultant balloon would be determined by how much black powder you loaded. Specifically by the number of moles of each of the components. Under standard temperature and pressure, one mole of a given material will produce 22.5 cubic feet of gas. Then, since the pressure inside the balloon would be greater than the ambient pressure (determined by the elasticity of the balloon's magic material), the gasses would simply leak back through the touch hole or nipple until the balloon deflates or is removed. And that's the truth. :thumbsup: