I was thinking about this for the past couple of weeks. Surley if you have a normal sized hand sling with a stick added onto the length it would throw a projectile further and faster due to the added leverage.

do you think the stick sling could have a triggered release?
is it just a small version of the staff sling

I wuold think that the staff sling would be heavy artillery ,  maybe not as accurate but capable if throwing  heavy rocks in an easy high arc,it seems to me that it is easier to increase the rock size to increase energy ..than it is to increase spead beyond a certain point .

Staffling is more difficult than you think.  But you can shoot very accurately if you know important factors.  1 mark your hand positions on the staff.  As with string walking when shooting with a bow, different hand positions on the stick result in different distances.  use the same heavy ammunition.  3 don't tear your stick over your shoulder.  It is best to be in a raised position above your head.  Your movement must be very even.  This is the only way the cord will always trigger immediately.  There are other things to look out for.  But that's enough for a beginner.  I saw original Fustibal in a collection in Vienna.  They all had different markings.  Scratched in the stick.  Oh, there's one more important point.  The length of the sling should only be half the length of the stick.  If the sling is longer, the rod can break.  This has happened to me twice already.  Very uncomfortable.  The stick should be about the size of yourself. Not bigger.  Use light sticks.  Willow or hazelnut are very suitable.  Likewise young ash.  In historical pictures you can see David with a staff.  This rod is slightly curved at the bottom.  It is believed to be a shepherd's staff.  Then the curvature would certainly be carried upwards.  I'm pretty sure these bars represent fustibals.

https://m.youtube.com/watch?v=2XogFAiB94I

The stick sling really didn't work out that well. The main problem is just that it does everything a hand sling does but worse Still kind of fun to mess around with but full staff slings are where it's at.

Very true. It was an accident that I made it really. It was just because I cut down the original staff sling and had the piece of wood spare.

The one I was using for the staff sling ammo is an Hungarian gas mask bag. Really cheap from a army surplus website (in fact I got two for the price of one and sent one to Curious_Aardvark in an exchange) and it works great. Hard wearing and holds a lot of ammo. The small one in the above picture I use for really small ammo and I made that myself from jute using nålbinding.