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 is an estimate; always confirm your load against a current published manual before loading.
Use MyLoads as a 257 Roberts +P velocity and pressure calculator: it estimates muzzle velocity, muzzle energy and the full chamber-pressure curve for your own powder, charge and barrel length, compared against the reference facts above.
Open the 257 Roberts +P in the MyLoads calculator →