When i had just discovered the internet and was still fumbling my way around like a one armed teen on his first bra, I tripped over this site about slinging.
 
It gave some facts and very limited info on the sling. But, being ever curious, i ventured on and found this:
 
A recipe for clay (ball) pellets. The way the Sumerians and Egyptians made them.
 
1). 3 to 4 parts river clay.
2). 1 part horse, camel or cow dung.
3). Mix the 4 parts together until the dung and clay are indistinguishable (homoginized)
From what i gathered, the clay and manure was placed in a container and trampled by foot until it was indistinguishable and the right consistency. Manure was added to harden the clay and make it more "brick-like" during firing.
4). Firing was accomplished by rolling and drying the pellets on gaurd duty. When they were sun-dried, these pellets would be placed in the bottom of the watch fire for the night shift gaurds, who would keep the fire going all night.
 
In the morning, the cooled down pellets would be removed and placed in containers for later use.
 
I remember the site saying that pellets, as these, had been excavated at various sites.
 
Some of these pellets were the size of tennis balls and were, aparrently thrown, using the 2-handed method. Much like an olympic hammer thrower would today.
 
Anyone know of this or have any info on clay and manure pellets.
 
The reason i ask is that i will go on leave soon and intend to make such pellets.

Sounds like a recipe for bricks, I think it'll work pretty well for sling ammo.

I'm new here so I'm a little curious.  
 
Anyone know whether the manure was dry or wet (freshly extruded) when it was mixed? Was manure used because of specific (special) properties of manure, especially if it was used when it was fresh or because straw materials may have been scarce?  
 
I don't know about clay used to make sling stones in this fashion but I do know adobe. Not the computer program, the bricks used to build homes and the like. I know adobe because I grew up making and using them and still do. My dad and I make and use clay ovens or "horno". When making your bricks you need chopped up straw as a binder. We chop it by hand with a machete.  
 
I can see where dried manure would be a readily available source of chopped straw especially in arid regions. You could cut out a whole process when making bricks and if making small items such as sling stones the finely "processed" straw would probably be ideal. I've toyed with using very dry cow manure for making brick but C'mon, do you really want to eat pies, corn or bread baked in an oven made partially of cow poo? I don't think so.

The micro fibers in horse,cow etc manure help binding the clay,as you guys have said.Like metal bars in concrete. But it's not a filthy material like y'all think.It's pretty much chopped vegetable fiber. And if you make an oven using it,it will burn out on the inside at least and will not affect your bread anyhow.Human manure is waaaaay more nasty,because the things humans eat.

be interested to see the original article greenman.  
 
Any idea where you read it ?
 
The idea that the guards were regular making their own sling ammo would be an indication that the sling enjoyed a much greater usage and popularity in egypt than - I suspect - most of us had realised.  
 
And rather you than me on making your own  

I recently picked up some clay at a pottery shop and on friday made up 32 sling stones. They're drying out on the porch now but because of this recent post I made a few using chopped up sisal as a binder just to see how they'd come out. I plan on getting my stones fired at the pottery shop once they're dry enough to do so. I won't fire my fiber stones at the pottery shop because the temperatures are so high but will toast them on the BBQ grill just to harden them up a bit. Now when I trade slings I will include one of my clay stones, replacing the river rock I've been sending. I think, when gifting a sling you should include a stone for good luck just like you would include money if you gifted a purse or wallet.