As I recall the shotgun Sabot Slug was developed by the French military/police following WW2.
Actual development stated during the war. They noticed that they had a problem in barricade/ambush situations where the 9mm just couldn't stop or even penetrate an vehicle body. And a standard slug wasn't accurate enough and didn't have enough velocity to really do a job on an engine block.
With hunters/shooters being what we are.... "Hey, whatcha know? An accurate slug round!"
The rest, as they say, is history.
zeprin