A measuring staff will make things clear. The flash was used twice with an interval of 1 msec. The white dot to the extreme left indicates the position of the bullet when flashing a first time. The second dot to the right indicates the position during the 2nd flash. So we can measure the exact speed of the bullet using the distance between these two dots and knowing that it took 1 msec between the two flashes. The distance of 15cm gives us a speed of 150 msec. So if an object is placed at 30cm from the barrel we should establish a delay of 2 msec to get the bullet exactly positioned in our photograph. Now we can determine the exact position of the bullet very carefully!
Another example is shooting through a piece of string with a weight hanging at its end. First an overvieuw of the settings:
--> see next image.