What is the powder burn or propellant combustion?

Powder burn or propellant combustion is the solid-to-gas conversion of the powder. The metric "z" defines the percentage of the burnt propellant when the projectile leaves the muzzle.

When optimizing a load through the appropriate selection of components, combustion preferably ends before the projectile passes the muzzle. Achieving more than 95-98% of burnt propellant is considered to be a fair load. This improves energy utilization, ensures consistent results, and influences the generation and appearance of the muzzle flash.