Bullet designs have to solve two primary problems. They must first form a seal with the gun’s bore. If a strong seal is not achieved, gas from the propellant charge leaks past the bullet, reducing efficiency. The bullet must also engage the rifling without damaging the gun’s bore. Bullets must have a surface which will form this seal without causing excessive friction. These interactions between bullet and bore are termed internal ballistics. Bullets must be produced to a high standard, as surface imperfections can affect firing accuracy.
The physics affecting the bullet once it leaves the barrel, is termed external ballistics. The primary factors affecting the aerodynamics of a bullet in flight are the bullet’s shape and the rotation imparted by the rifling of the gun barrel. Rotational forces stabilize the bullet gyroscopically as well as aerodynamically. Any asymmetry in the bullet is largely canceled as it spins. With smooth-bore firearms, a spherical shape was optimum because no matter how it was oriented, it presented a uniform front. These unstable bullets tumbled erratically and provided only moderate accuracy, however the aerodynamic shape changed little for centuries. Generally, bullet shapes are a compromise between aerodynamics, interior ballistic necessities, and terminal ballistics requirements. Another method of stabilization is for the center of mass of the bullet to be as far forward as is practical, which is how the Minié ball and the shuttlecock are designed. This allows the bullet to fly front-forward by means of aerodynamics.
See Terminal ballistics and/or Stopping power for an overview of how bullet design affects what happens when a bullet impacts with an object. The outcome of the impact is determined by the composition and density of the target material, the angle of incidence, and the velocity and physical characteristics of the bullet itself. Bullets are generally designed to penetrate, deform, and/or break apart. For a given material and bullet, the strike velocity is the primary factor determining which outcome is achieved.