Estimate chamber pressure and muzzle velocity for the 257 Roberts +P 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.
| Maximum average pressure (MAP) | 58,000 psi |
|---|---|
| Nominal case capacity | 56.0 gr H₂O |
| Groove / bullet diameter | 0.257 in (6.53 mm) |
Muzzle velocity
Peak chamber pressure (interval, ±10% burn-rate)
nominal is 66% of the 58,000 psi maximum average pressure
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 257 Roberts +P in the MyLoads calculator →