What is the bullet jump and when to use?

CAUTION – Bullet Jump Function 

The Bullet Jump function models the potential movement of a bullet caused by the primer impulse before the powder begins producing significant pressure. In this context, bullet jump refers to the distance the bullet travels before contacting the rifling due to primer energy alone, prior to the main powder ignition phase. 

Studies and experimental observations indicate that the primer impulse alone can sometimes move the bullet forward in the case, potentially seating it closer to—or even against—the rifling before the powder charge begins generating substantial pressure. 

This effect may occur more frequently in cartridges with straight-wall cases, minimal shoulder angles, large bullet diameters, or relatively slow-burning powders. 

SAFETY WARNING 

THIS FUNCTION REDUCES THE DISPLAYED PRESSURE VALUES AND SHOULD ONLY BE USED WITH EXTREME CAUTION. If your firing tests show significantly lower measured velocities than the simulated values, one possible explanation is that the bullet is partially unseated by the primer impulse before effective powder ignition occurs. 

When this happens, the bullet moves forward before pressure builds, effectively increasing the combustion chamber volume. The result can be lower peak pressure, delayed pressure rise, and reduced muzzle velocity compared to the calculated values. 

Users can test for this behavior experimentally. One method is to load 3–5 cartridges at the standard COAL and another 3–5 cartridges with the bullet seated longer so that it nearly touches or lightly contacts the rifling, while keeping the powder charge identical.