For example, let's take two 12ga, 1 1/8 oz lead target loads. MV of the fast one is 1400 fps; the slow one, 1100 fps. At 20 yards, fast load velocity is 951 fps; slow load, 790 fps. Flight time to 20 yards is .0511 seconds for the fast load; .0630 for the slow load. At 40 yards, velocities are 679 fps vs 574 fps. Flight time, fast vs slow: .1265 seconds vs .1529 seconds. (Source: John Taylor's "Shotshells and Ballistics".) Which means, even though the difference in velocity between fast and slow loads has been reduced, you need more forward allowance to compensate at 40 yards than you do at 20 yards.
Say we have a 40 mph (60 fps) 90 degree crossing target a 20 yds. Target movement for the high vel load is 0.0511 sec X 60 fps = 3.07 ft and the low vel load is 0.0630 X 60 = 3.78. The low vel needs 0.71 ft = 8.5 inches more forward allowance (23% more) for the same pattern placement. At 40 yards: 0.1265 X 60 = 7.59' and 0.1529 X 60 = 9.174, so 9.174' - 7.59' = 1.584' = 19" additional forward allowance (21% more).
DDA
