What is "internal ballistics"

Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile.

In guns internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The study of internal ballistics is important to designers and users of firearms of all types, from small-bore rifles and pistols, to high-tech artillery.

For rocket-propelled projectiles, internal ballistics covers the period during which a rocket motor is providing thrust.