The Firing Sequence
Internal ballistics is the study of events between ignition and the instant the projectile exits the muzzle. It examines how burning propellant generates rapidly rising gas pressure and how that pressure accelerates the projectile along the bore, all in a span of roughly one to two thousandths of a second.
Pressure and Acceleration
When the primer ignites the propellant, the powder burns and releases expanding gas that raises chamber pressure extremely fast. That pressure acts on the base of the projectile, driving it forward. As the projectile moves and the space behind it enlarges, pressure peaks early and then declines even as the projectile keeps accelerating, because gas is still pushing on it the entire length of the bore.
The Pressure Curve
Chamber pressure is not constant; it follows a sharp curve that rises to a peak shortly after the projectile starts moving, then tapers as the burning gas expands into the growing volume behind the projectile. The area under this pressure-versus-position curve represents the work done on the projectile and largely sets its muzzle velocity.
Propellant Burn Rate
Powders are engineered with different burn rates, and the burn rate must be matched to the cartridge and barrel. A fast-burning powder reaches peak pressure quickly and suits short barrels and small cases, while a slower powder sustains pressure longer to keep accelerating a projectile down a long barrel without exceeding safe peak pressure.
Barrel Time and Barrel Length
Barrel time is the brief interval the projectile spends traveling through the bore. A longer barrel gives the expanding gas more distance to keep pushing, generally yielding higher muzzle velocity, up to the point where friction and falling pressure stop adding meaningful speed. This is why cutting a barrel shorter usually lowers velocity.
Worked Example
Picture a rifle cartridge where peak pressure reaches its maximum after the projectile has moved only about an inch. Pressure might spike to tens of thousands of pounds per square inch, then fall to a fraction of that by the muzzle, yet the projectile continues gaining speed the whole way because net forward force remains positive. Over perhaps 24 inches of travel in roughly 1.2 milliseconds, the projectile accelerates from rest to its full muzzle velocity.
A Common Misconception
It is often assumed that peak chamber pressure occurs at the muzzle or that the projectile is fastest where pressure is highest. In fact pressure peaks very early, near the start of travel, while velocity keeps climbing all the way to the muzzle. Peak pressure and peak velocity happen at different points, which is exactly why the pressure curve, not a single number, describes internal ballistics.