You may have difficulty stuffing enough WC872 into the case to achieve even normal velocity. Expect lots of unburned (and wasted) granules since this 20mm cannon powder burns pretty slow (in most lots, though some may be quicker).
Here are some QuickLOAD estimates (not real data!) for H870 in .30-40 -
Cartridge : .30-40 Krag Bullet : .308, 220, Sierra SPRN 2180 Cartridge O.A.L. L6: 3.089 inch or 78.46 mm Barrel Length : 30.0 inch or 762.0 mm Powder : Hodgdon H870
Predicted data by increasing and decreasing the given charge, incremented in steps of 2.941% of nominal charge. CAUTION: Figures exceed maximum and minimum recommended loads !
Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time % % Grains fps ft.lbs psi psi % ms
-11.8 88 40.54 1616 1276 16135 3951 63.7 2.534 -08.8 91 41.90 1675 1370 17466 4224 66.2 2.508 -05.9 94 43.25 1736 1473 18932 4504 68.6 2.418 -02.9 97 44.60 1800 1583 20551 4787 71.1 2.330 +00.0 100 45.95 1866 1701 22341 5072 73.5 2.243 +02.9 103 47.30 1934 1827 24329 5358 76.0 2.158 +05.9 106 48.65 2004 1962 26538 5641 78.4 2.075
Note the absurd "Burnt %" figures.
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