driving the projectile into a "forcing cone" that provides resistance as it compresses the projectile obturation ring. Kinetic friction The bullet must tightly fit the bore to seal the high pressure of the burning gunpowder. This tight fit results in a large frictional force. The friction of the bullet in the bore does have a slight impact on the final velocity, but that is generally not much of a concern. Of greater concern is the heat that is generated due to the friction. At velocities of about , lead begins to melt, and deposit in the bore. This lead build-up constricts the bore, increasing the pressure and decreasing the accuracy of subsequent rounds, and is difficult to scrub out without damaging the bore. Rounds, used at velocities up to , can use wax lubricants on the bullet to reduce lead build-up. At velocities over , nearly all bullets are jacketed in copper, or a similar alloy that is soft enough not to wear on the barrel, but melts at a high enough temperature to reduce build-up in the bore. Copper build-up does begin to occur in rounds that exceed , and a common solution is to impregnate the surface of the bullet with molybdenum disulfide lubricant. This reduces copper build-up in the bore, and results in better long-term accuracy. Large caliber projectiles also employ copper driving bands for rifled barrels for spin-stabilized projectiles; however, fin-stabilized projectiles fired from both rifle and smoothbore barrels, such as the APFSDS anti-armor projectiles, employ nylon obturation rings that are sufficient to seal high pressure propellant gasses and also minimize in-bore friction, providing a small boost to muzzle velocity. The role of inertia In the first few centimeters of travel down the bore, the bullet reaches a significant percentage of its final velocity, even for high-capacity rifles, with slow burning powder. The acceleration is on the order of tens of thousands of gravities, so even a projectile as light as can provide over of resistance due to inertia. Changes in bullet mass, therefore, have a huge impact on the pressure curves of smokeless powder cartridges, unlike black-powder cartridges. The loading or reloading of smokeless cartridges thus requires high-precision equipment, and carefully measured tables of load data for given cartridges, powders, and bullet weights. Pressure-velocity relationships Energy is imparted to the bullet in a firearm by the pressure of gases produced by burning propellant. While higher