Interesting video
Getting enough accuracy is the main problem with slings
I played alot with differents types of slings and different techniques but never attained a skill that made me trust my shots.
Ammonitions are cheap (even if a careful selection drastically improves the result) but even in a survival situation would you risk to scare your potential lunch with a catastrophic shot?
I'd rather invest my energy in traps or maybe into making a spear/javelin
Apache style was the one that worked best for me in terms of accuracy
Nedless to say is one of the less powerfull techniques (in terms of speed/energy)