How does ApexLOAD PRO reflect zero bullet jump?
When you set a bullet jump of 0 mm in ApexLOAD PRO — indicating that the bullet is seated against the rifling lands — the calculator does not treat the load the same as one with normal free travel. The internal ballistic effect of zero jump is significant: peak chamber pressure rises substantially because the bullet must overcome rifling engraving resistance from a standstill. (For the underlying physics, see What is the impact of zero bullet jump on pressure and muzzle velocity?.)
This article describes how ApexLOAD PRO adjusts its calculation to account for this effect.
The shot start pressure adjustment
Most internal ballistics calculators do not detect bullet jump automatically. They require the user to manually indicate this situation. ApexLOAD PRO flags the risk of having the bullet seated against the lands. The user can then confirm that the jump is 0 mm and ApexLOAD applies a multiplier of approximately 2.5× to the baseline shot start pressure for the selected bullet.
Because monolithic copper bullets resist engraving more than lead-core or FMJ projectiles, the baseline shot start pressure already varies by construction, and the 2.5× multiplier produces correspondingly different absolute values.
ApexLOAD PRO's bullet database tags each projectile with its construction class and applies the appropriate baseline automatically — you don't need to enter it manually. The resulting calculated peak pressure rises by approximately 5–10% compared to the same load with normal jump, and ApexLOAD PRO's recommended maximum charge weight decreases accordingly. This is intentional — we'd rather give you a slightly conservative maximum that keeps you inside the safe pressure envelope than an optimistic one that doesn't.
What you should do as a reloader
If you load to the lands by choice:
Set the bullet jump in ApexLOAD PRO to 0 mm so the calculation reflects the real geometry.
Start your load development at least 5% below the calculated maximum charge weight, not at the maximum.
Work up incrementally and watch for traditional pressure signs (cratered primers, ejector marks, sticky bolt lift, unusual recoil). At zero jump these matter more than calculated values.
Never copy a maximum charge developed at normal jump and apply it unchanged to a jammed load.
If you don't deliberately load to the lands but suspect your bullet might be very close, long-ogive projectiles or magazine-length-constrained loads can put you closer than you think — measure your actual jump with a comparator and an OAL gauge. ApexLOAD's results are only as accurate as the jump value you enter.
A note on the underlying data
The 2.5× multiplier is based on published research and aggregated community observations. The exact physical relationship between jump and shot start pressure is not perfectly linear, and there is real bullet-to-bullet, lot-to-lot, and rifle-to-rifle variation. ApexLOAD PRO's adjustment is a calibrated approximation that errs on the conservative side — accurate enough to keep your load development inside safe bounds, but not a substitute for incremental work-up and chronograph verification.