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338-378 Weatherby Magnum Load Data & Ballistics Calculator

Estimate chamber pressure and muzzle velocity for the 338-378 Weatherby Magnum with the free, in-browser MyLoads internal-ballistics simulator. The figures below are illustrative estimates from a physics model — not a recommended recipe. Always cross-check a current published loading manual.

338-378 Weatherby Magnum reference

Maximum average pressure (MAP)4,400 BAR
Nominal case capacity127.5 gr H₂O
Groove / bullet diameter0.338 in (8.59 mm)

Example simulation illustrative

Bullet
Hornady 0.338 250 gr BTHP Match
Powder
Vihtavuori N-170
Charge
90 gr
Barrel
28 in

Muzzle velocity

2,700 fps

Peak chamber pressure (interval, ±10% burn-rate)

41,116 – 52,809 – 65,638 psi

nominal is 83% of the 63,817 psi maximum average pressure

This sample load runs near the maximum average pressure for the 338-378 Weatherby Magnum (about 83% of MAP at the nominal estimate; the upper interval can exceed it). It is shown only to illustrate the simulator — reduce the charge and work up from your manual's start load.

Illustrative example — every figure here is an estimate and can be wrong. Always consult a current published loading manual, and never trust these numbers on their own.

Open the 338-378 Weatherby Magnum in the MyLoads calculator →

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