The ideal gas law is PV=nRT. P is pressure, V is Volume, n is number of moles (which would be related to grains of powder), R is a constant and T is temperature. It applies only to an ideal gas, or to approximate a real gas. But is does not account for many parameters and characteristics. It is most often used for single molecule gases (which we would not have) at low pressures and high temperatures. But it still can be used to grasp a fundamental understanding of the relationships between the 4 variables.
You would be better off looking at choked flows and supersonic flow theory. There you would gain the understanding those who say that the form factor (bottlenecked case) does not result in a pressure increase can not grasp. In very simple terms, you have water flowing from a hose at full diameter. If you restrict the opening through which the water is flowing...what happens? The pressure IN the hose goes UP and the velocity of the water flowing OUT of the hose goes UP (it is propelled further). If you want the water to have the same velocity without restriction, you must SIGNIFICANTLY increase the volume of water you put through the hose. The "cost" of the restriction is a LOT less than the cost of a higher water volume to get something "far away" wet. Also energy goes as velocity squared, so that higher velocity water does more work than the lower velocity water. You all know this and see this when you wash your car.
