pressures produce higher velocities, pressure duration is also important. Peak pressure may represent only a small fraction of the time the bullet is accelerating. The entire duration of the bullet's travel through the barrel must be considered. Peak vs area Energy is defined as the ability to do work on an object; for example, the work required to lift a one-pound weight, one foot against the pull of gravity defines a foot-pound of energy (One joule is equal to the energy needed to move a body over a distance of one meter using one newton of force). If we were to modify the graph to reflect force (the pressure exerted on the base of the bullet multiplied by the area of the base of the bullet) as a function of distance, the area under that curve would be the total energy imparted to the bullet. Increasing the energy of the bullet requires increasing the area under that curve, either by raising the average pressure, or increasing the distance the bullet travels under pressure. Pressure is limited by the strength of the firearm, and duration is limited by barrel length. Propellant design Propellants are carefully matched to firearm strength, chamber volume and barrel length, and to bullet material, weight and dimensions. The rate of gas generation is proportional to the surface area of burning propellant grains in accordance with Piobert's Law. Progression of burning from the surface into the grains is attributed to heat transfer from the surface of energy necessary to initiate the reaction. Smokeless propellant reactions occur in a series of zones or phases as the reaction proceeds from the surface into the solid. The deepest portion of the solid experiencing heat transfer melts and begins phase transition from solid to gas in a foam zone. The gaseous propellant decomposes into simpler molecules in a surrounding fizz zone. Endothermic transformations in the foam zone and fizz zone require energy initially provided by the primer and subsequently released in a luminous outer flame zone where the simpler gas molecules react to form conventional combustion products like steam and carbon monoxide. The heat transfer rate of smokeless propellants increases with pressure; so the rate of gas generation from a given grain surface area increases at higher pressures. Accelerating gas generation from fast burning propellants may rapidly create a destructively high pressure spike before bullet movement increases reaction volume. Conversely, propellants designed