Progressive burn rate. Most propellant is not solid, but it comprised of many small pellets. The shape of the individual pellets will determine the type of burn rate. Pellet type propellant shapes. The type of propellant will alter the shape of the pressure vs. position curve.
Progressive pellets raise the pressure more slowly than degressive propellants.
For this reason, the peak pressure is often less. On the other hand, degressive propellants accelerate the projectile more rapidly in the initial portion of the barrel, while progressive propellants can reach higher exit velocities. Progressive vs. degressive propellants. Which type of propellant to use depends on the application. If the barrel cannot be made very long, it is better to use a degressive propellant to achieve the maximum exit velocity in a limited distance.