Load Specifications
Load data in caliber .256 Mag. Gibbs with .257 | 100gr | Impala RNFP KO bullet and 28 powders. The key underlying parameters are shown in the table above. The recommended powders for this cartridge and bullet combination are Somchem S365, Alliant Reloder-16 and Norma URP.
Important Notice: DO NOT go below Minimum Charge or above Maximum Charge. Since we have no control over the equipment and components used, any responsibility and liability for the results achieved is rejected.
Only powders/loads that match the current filter criteria are displayed: max pressure of a load ≤ 90% of max. standardized pressure, fill rate ≥ 80% and propellant combustion ≥ 95%. Use the sliders above the tables to adjust these thresholds.
Charge: Powder weight in grains. Start with the minimum load and work up carefully.
FR (Fill Rate): Case fill as a percentage. Values above 100% mean compressed loads.
MaxP / %: Maximum chamber pressure in bar and as a percentage of the standardized maximum for this caliber.
V(0): Muzzle velocity in feet per second (fps) at the standardized barrel length.
z (Powder Burn): Estimated propellant combustion percentage at bullet exit.
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Max Load - Barrel Length: 24"
| Powder | Charge | FR | MaxP | % | V(0) | z |
|---|---|---|---|---|---|---|
| Accurate 4350 | <<locked>> | 90% | 44,396 psi | 90% | 2953 fps | 97.4% |
| ADI AR 2208 | <<locked>> | 82% | 43,294 psi | 88% | 2854 fps | 97.6% |
| Alliant Reloder- TS 15.5 | <<locked>> | 82% | 43,381 psi | 88% | 2884 fps | 98.9% |
| Alliant Reloder-16 | <<locked>> | 90% | 42,917 psi | 87% | 2949 fps | 97.3% |
| Alliant Reloder-17 | <<locked>> | 86% | 42,931 psi | 87% | 2979 fps | 98.7% |
| Alliant Reloder-23ᴾ | <<locked>> | 100% | 42,395 psi | 86% | 2963 fps | 98.6% |
| Alliant Reloder-26 | <<locked>> | 98% | 44,309 psi | 90% | 3061 fps | 97% |
| Bofors RP3 | <<locked>> | 82% | 43,845 psi | 89% | 2900 fps | 99.1% |
| Hodgdon H380 | <<locked>> | 82% | 44,164 psi | 90% | 2894 fps | 96.7% |
| Hodgdon Hybrid 100V | <<locked>> | 92% | 42,424 psi | 86% | 2890 fps | 98.5% |
| Hodgdon TrailBossᴾ | <<locked>> | 100% | 35,911 psi | 73% | 1929 fps | 100% |
| IMR 4831 | <<locked>> | 90% | 42,250 psi | 86% | 2890 fps | 96.9% |
| Norma URP | <<locked>> | 90% | 43,569 psi | 88% | 2940 fps | 97.1% |
| ReloadSwiss RS 14ᴾ | <<locked>> | 100% | 36,463 psi | 74% | 1972 fps | 100% |
| ReloadSwiss RS 60 | <<locked>> | 84% | 42,714 psi | 87% | 2986 fps | 99.9% |
| ReloadSwiss RS 62 | <<locked>> | 90% | 43,700 psi | 89% | 2966 fps | 96.9% |
| ReloadSwiss RS 70 | <<locked>> | 90% | 43,366 psi | 88% | 2972 fps | 96.1% |
| Rottweil R 903 | <<locked>> | 84% | 43,918 psi | 89% | 2894 fps | 98.9% |
| SNPE Vectan SP 11 | <<locked>> | 84% | 44,237 psi | 90% | 2936 fps | 98.4% |
| SNPE Vectan Tubal 5000 | <<locked>> | 82% | 42,859 psi | 87% | 2766 fps | 100% |
| Somchem S355 | <<locked>> | 84% | 44,208 psi | 90% | 2894 fps | 98.8% |
| Somchem S365 | <<locked>> | 92% | 41,887 psi | 85% | 2946 fps | 99.5% |
| Vihtavuori N150 | <<locked>> | 84% | 42,322 psi | 86% | 2825 fps | 99.4% |
| Vihtavuori N160 | <<locked>> | 88% | 42,612 psi | 86% | 2877 fps | 96.7% |
| Vihtavuori N550 | <<locked>> | 88% | 42,409 psi | 86% | 2887 fps | 96.5% |
| Vihtavuori N555 | <<locked>> | 98% | 44,092 psi | 89% | 2995 fps | 98% |
| Winchester 6.5 StaBALL | <<locked>> | 88% | 42,540 psi | 86% | 2854 fps | 98.7% |
| Winchester StaBALL HDᴾ | <<locked>> | 100% | 38,392 psi | 78% | 2966 fps | 98.5% |
Recommended = one of the top powders for this cartridge and bullet combination.
Start Load - Barrel Length: 24"
| Powder | Charge | FR | MaxP | % | V(0) | z |
|---|---|---|---|---|---|---|
| Accurate 4350 | <<locked>> | 84% | 36,608 psi | 74% | 2776 fps | 95% |
| ADI AR 2208 | <<locked>> | 80% | 40,625 psi | 82% | 2795 fps | 97% |
| Alliant Reloder- TS 15.5 | <<locked>> | 80% | 40,770 psi | 83% | 2828 fps | 98.5% |
| Alliant Reloder-16 | 40.6 gr | 86% | 38,189 psi | 77% | 2835 fps | 95.7% |
| Alliant Reloder-17 | <<locked>> | 80% | 35,607 psi | 72% | 2792 fps | 96.6% |
| Alliant Reloder-23 | <<locked>> | 92% | 33,634 psi | 68% | 2733 fps | 95.6% |
| Alliant Reloder-26 | <<locked>> | 94% | 39,044 psi | 79% | 2933 fps | 95.1% |
| Bofors RP3 | <<locked>> | 80% | 41,118 psi | 83% | 2844 fps | 98.6% |
| Hodgdon H380 | <<locked>> | 80% | 41,481 psi | 84% | 2838 fps | 96% |
| Hodgdon Hybrid 100V | <<locked>> | 84% | 33,475 psi | 68% | 2674 fps | 95.8% |
| Hodgdon TrailBoss | <<locked>> | 80% | 27,079 psi | 55% | 1729 fps | 100% |
| IMR 4831 | <<locked>> | 86% | 37,434 psi | 76% | 2782 fps | 95.4% |
| Norma URP | 41.2 gr | 86% | 38,580 psi | 78% | 2825 fps | 95.6% |
| ReloadSwiss RS 14 | <<locked>> | 80% | 27,383 psi | 56% | 1768 fps | 100% |
| ReloadSwiss RS 60 | <<locked>> | 80% | 37,681 psi | 76% | 2867 fps | 99.3% |
| ReloadSwiss RS 62 | <<locked>> | 86% | 38,450 psi | 78% | 2848 fps | 95.3% |
| ReloadSwiss RS 70 | <<locked>> | 88% | 40,553 psi | 82% | 2904 fps | 95.1% |
| Rottweil R 903 | <<locked>> | 80% | 38,653 psi | 78% | 2779 fps | 97.8% |
| SNPE Vectan SP 11 | <<locked>> | 80% | 39,001 psi | 79% | 2818 fps | 97.1% |
| SNPE Vectan Tubal 5000 | <<locked>> | 80% | 40,480 psi | 82% | 2720 fps | 100% |
| Somchem S355 | <<locked>> | 80% | 39,233 psi | 80% | 2782 fps | 97.7% |
| Somchem S365 | 38.6 gr | 80% | 29,283 psi | 59% | 2608 fps | 95.5% |
| Vihtavuori N150 | <<locked>> | 80% | 37,275 psi | 76% | 2710 fps | 98.5% |
| Vihtavuori N160 | <<locked>> | 84% | 37,768 psi | 77% | 2769 fps | 95.2% |
| Vihtavuori N550 | <<locked>> | 86% | 39,827 psi | 81% | 2828 fps | 95.7% |
| Vihtavuori N555 | <<locked>> | 90% | 34,476 psi | 70% | 2769 fps | 95% |
| Winchester 6.5 StaBALL | <<locked>> | 80% | 33,069 psi | 67% | 2625 fps | 95.8% |
| Winchester StaBALL HD | <<locked>> | 94% | 32,053 psi | 65% | 2759 fps | 95.2% |
Recommended = one of the top powders for this cartridge and bullet combination.
ᴾ possible press load - work up slowly to this load while watching for signs of excessive pressure.
This overview compares 28 powders for .256 Mag. Gibbs loaded with the .257 | 100gr | Impala RNFP KO. Alliant Reloder-26 delivers the highest muzzle velocity at 3,061 fps; across all powders the maximum-charge window runs from 1,929 fps to 3,061 fps. Alliant Reloder-23 gives the best case fill at 100 %, and 28 of the 28 powders fall inside the usable 80–100 % fill window.
Relative to the caliber's maximum pressure (340 MPa), the top charges use 73–90 %, leaving at least 10 % headroom. The 100.0 gr projectile in 0.257" calls for a 1:10.0 twist; slower twists will usually not stabilise this weight. Alliant Reloder-23, Hodgdon TrailBoss, ReloadSwiss RS 14 reach 100 % fill or more at maximum charge, so these are compressed loads: they need a long drop tube to seat cleanly and react sensitively to the case capacity of your own brass lot. Somchem S365, Alliant Reloder-16, Norma URP rank at the top here: for .256 Mag. Gibbs with the .257 | 100gr | Impala RNFP KO they combine a high case fill with complete burn and sit in the upper velocity band. Always begin at the start load and work up in small increments while checking case and primer for pressure signs.