Seemed to me those 7/8oz. Foster (shuttlecock) Style shotgun slugs, due to their poor BC, had a maximum range of something like 600 yards. That was back in the 60's - an article in "Guns and Junk "Ammo" magazine, spouting WHY there were shotgun seasons in builtup areas and why ML's were also allowed - due to the short range of round balls.
Sometimes, 'Better' isn't better.
The first thing mfgr's did, was to increase the weight of the Foster slug, from 7/8oz. to 1oz and 1 1/4oz to make them 'better' , thus increasing their range making them 'less safe' in shotgun-only areas. Then the ML makers brought out slugs for those, when prior to that, all the old timers still in the game and the new shooters to the sport, were using round balls. The slugs increased the ranges and special weapons seasons, shotgun and ML's, started being closed - that was the gist of the story's continuance into the late 60's, easrly 70's.
Give him a chance and man will screw it up as he pushes the rules as hard as possible - used to call that cheating and as we know, cheaters never prosper.