With a little practice I have got the reliable reload time down to 12 seconds, so the rate of fire is half that of a slinger or an archer.  This might be what prevented the greater adoption of the weapon.
 
However, staff slings arent especially fast and they were widely used.  I still reckon that a two man staff-cestros team with one doing the throwing and the other doing the loading would have been a formidable unit on a Roman battlefield.
 

It looks fine to me! The originals had loads of iron on them so dont worry about the weight. Take it somewhere big and empty and let rip!
 
p.s. I can strongly recommend bright colours on the tail because you will be amazed how hard this thing can be to find sometimes.

Just a thought: but given such an impressive reproduction and the slinging book being compiled, maybe we could throw this in too

This is my attempt to describe how to speed load the cestrosphendone. It wont make much sense unless you have one and try to follow these steps.
 
The dart is in your left hand pointing up, the loop is in your right hand
 
Bring the loop behind the dart
 
With your left thumb, bend the loop around the dart so the tip of the loop is in front. Hold the loop in place with the same thumb.
 
With your right hand, take the peg. Come behind the furthest side of the loop, the knotted side, and push the tip of the peg through the loop towards yourself
 
Bring the tip of the peg past the tip of the loop, then turn the peg and reverse direction, hooking into the tip of the loop and bringing it back through the far side of the loop.
 
Turn the peg so it is pointing towards the tip of the dart, grasp the release cord and pull tight.
 
 

Broomstick shafts. Fused shopping bag fletching. Iron tie wire hafting at the tips. The broadhead is made from a stainless table spoon. The soon to be spike is 1/2" diameter cold rolled steel.
The broadhead dart is 201g. and the current weight of the top one is 323g.
Even unsharpened, the top one has some devastating penetration. So much fun.

Good work there, those are some seriously meaty darts.  Anyone got any more?

Congratualtions, you guys found a way to make Lawn Darts even more dangerous.  
 
Awesome weapon.
 
Perpetualstudent, great minds must think alike. I sent a message to David less that a week ago about the Guide. Even if it's not him that writes it, there will definitely be a kestros section.

This release really has had me thinking, and I've been carrying one since the beginning of this thread.  In some respects,  I consider it to be as versatile as a classic sling... it can hold a lot of things that an average EDC sling would be unable to pouch.  You can easily deliver a bottle should you find the need to do such a thing *cough cough*.  I've been having a blast chucking new things that I never before considered as ammunition.