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Op Rod velocity is directly proportional to the Impulse imparted to it. Conservation of momentum then immediately makes relative gas mass at the port every bit as much an issue as the port pressure. (!) As you can see from the pic comparing between the two "good" powders and the "no-no" 4350 necessary for equal velocities, port pressure is only 7% different for the slow 4350 powder from that of the IMR4064 "standard," but the resulting OpRod momentum is three times that. (!) (again)
I have read Parashooter's explanation and am puzzling over it. .