Ballistics
Internal ballistics is about the motion of projectile inside barrel (tube part of gun). This motion depends on the pressure of gas, which is created by burning gunpowder. The pressure is depends on (is related to) the amount of gunpowder in the bullet, the type of gunpowder, size of pieces of gunpowder, free space behind a bullet and outside temperature. This motion also depends on the size and weight of bullet. The most important number from internal ballistics for performance is the muzzle velocity (projectile speed from the front edge of barrel).