The sling was there when ‘The Grand Old Duke of York” marched his 10,000 men up and down again across the Netherlands and later, when Wellington’s riflemen harried the French in the Peninsula Campaign the sling was again along for the ride.
American sharpshooters in the Civil war had slings, but preferred to rest their rifles across a branch or a convenient log.
After these two agreements are made the rules go off in their own directions.
The international shooter’s rule deals with size, the sling may be no wider than 40mm (1.57 inches).
On the other hand the service rifle shooter must have the sling attached at the sling swivels with the sling placed either on the bottom or to the left or right of the magazine on an M-14 or M-16.