Observed velocity is the average velocity over 20 yds (18,3m) from muzzle ie 1250 mv = @1050 ov.
For practicle purpose as we are discussing spherical balls of lead with a fairly constant resistance(in small game sizes 2-3mm d) it will be found that this is usually the same as the actual velocity at 9yds (8,25m)

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745