When Burrard spoke of velocity he was generally using the standard "Observed Velocity" which was the average over 20 yds. The standard load of his day of 3 drams & 1 1/16oz shot had an observed velocity of 1050 fps. Converted to MV it was about 1300 fps. That 1150 velocity would have been over 1400 fps & likely pushing that 1450 fps. This was normally stated for #6 shot. If different sizes of shot were given the same MV, they would have different OV, larger shot being higher than smaller shot.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra