My best WAG.
The photo exposure began approximately 20 thousandths of a second after stuff left the barrel.
The duration of the camera exposure is approximately 1/1000 second. Point a1 is the location of the wad trailing edge when the exposure began. a2 is the leading edge of the wad at end of the exposure. Point b1 is where the slightly expanded fur ball of shot was when the exposure began. The distance between a1 and b1 is how far the shot was removed from the wad when the exposure began.
During the exposure the wad and shot were traveling at approximately 1,000-1,200 fps and would have traveled approximately 1-1.2 feet during the exposure which roughly appears to be the case in the photo when you extrapolate the length of the gun at approximately 4 feet.
The streak leading from the bore to a1 is a trail of “ejecta stuff” following the wad, hanging very momentarily in the still night air, highlighted by the floodlights /dark background and occurred prior to the photo exposure being taken.
b1-b2 is the shot “cloud” captured approximately 16-20 feet from the gun during 1/1000 second of flight. Approximately 2 feet in diameter and only beginning to transition from a ball of shot into a string several feet long.
a1-a2 is the flight of the wad recorded at a typical wad diameter of .75”, moving about a foot through the air and trailing the shot approximately 2’.
