This page reproduces the gun jump data from Ordnance Pamphlet 1100, the range-table pamphlet for the 16″/50 caliber Mark 7 gun carried by the Iowa-class battleships. Gun jump accompanies all U.S. Navy range tables for the gun the tables are computed for.
What gun jump is. When a gun fires, the projectile does not leave the muzzle traveling exactly along the axis the barrel pointed at rest. As the round accelerates down the bore, the barrel whips and droops and the whole mount reacts to the shock of firing, so the actual line of departure differs slightly from the measured angle of elevation. That angular difference — measured on the firing range from many rounds — is the gun jump. It is small but consistent, and because it repeats from shot to shot it can be measured once and built permanently into the ballistic solution.
The 16″/50 value. Gun jump for the 16″/50 is minus 9.7 minutes of angle — that is, the projectile departs 9.7 minutes of arc below the angle of elevation to which the gun is laid. The minus sign means the round leaves low relative to where the barrel points. This 9.7-minute correction was already taken into account when the OP-1100 range tables were computed, so the pointer and the fire-control solution need make no separate allowance for it; the tabulated ranges are correct as printed. The scanned OP-1100 page below records the figure as issued by the Bureau of Ordnance.
