Pattern the gun is the best way. If the shot string is above the speed of sound then the shot is going against a near vertical "wall" of compressed air. The shot string will close and slow very rapidly. Even the fastest load is subsonic somewhere around 20 yards. Then the normal flat plate drag laws associated with subsonic flight take over. That is usually an expansion of one inch per yard of travel. It's extremely hard to calculate because the shot coming from the barrel is very irregular and will make for a large and irregular bow wave. The speed of sound varies with the temperature so the the time spent for the shot to remain supersonic can vary. You are dealing with an irregular vehicle (shot group) and a quick change of temperature change from shotgun barrel to ambient traveling in the trans sonic speed range. Given the variables patterning the gun is the best way to get the information you want.
Last edited by pooch; 03/04/17 01:20 PM.
