.300 Whisper is one of 607 cartridges supported by the IBC calculator. Pick a load from your reloading manual, enter it, and the calculator predicts velocity and peak pressure, then gives a verdict against the cartridge's pressure ceiling. It is a comparison tool for working between loads you already trust, not a source of load data.
| Max average pressure | 3,700 bar ≈ 53,650 psi |
|---|---|
| Case capacity (water) | 25.2 gr H₂O (1.63 cm³) |
| Case length | 34.9 mm (1.374″) |
| Max overall length | 57.0 mm (2.244″) |
| Bore diameter | 7.820 mm (0.308″) |
| Standard | CIP |
Catalogued reference values the calculator works from. No charge, velocity, or pressure data appears on this page; loads come from your manual.
IBC checks itself against a corpus of published loads from printed reloading manuals: the engine is run on each printed load and compared with the printed result. 8 of those loads are .300 Whisper loads. On them, the model did this:
| Loads tested | 8 |
|---|---|
| Median velocity error | -5.0% (negative means the model reads slow) |
| Mean absolute velocity error | 4.6% |
| Within ±10% of printed velocity | 100.0% |
What these numbers are good for: judging how much confidence to put in a velocity comparison between two candidate loads for this cartridge. What they are not: a promise about any single load, and not load data. Charge weight, seating depth, powder lot, and your rifle all move the real result.
Open the calculator, pick .300 Whisper from the cartridge list, and enter a load from your current reloading manual. Compare variants: bullet weights, powders, charge steps. Start 10% below the manual's maximum charge and work up, watch the verdict on every rung, and check real pressure signs at the bench. The free tier covers single-load predictions and the first steps of a work-up ladder; PRO unlocks the full ladder and the charts.
Open the calculatorAccuracy figures on this page were computed from the published-manual validation corpus (9796 loads across 155 cartridges) with engine v1.1.14 (corpus build da7ac091). Page generated 2026-11-11. Method and provenance: methodology & accuracy.