I know it has no real bearing on what they are - I just like the name, plus it's what jaegoor calls them and they're his invention  

Popped into town yesterday. picked up a small plastic funnel for £0.65 and two packets of balloons with 30 baloons in each pack for £2.
So for a grand total of £2.65 (approx $4 or 3 euros)and some sand (a bag of sand can be 'borrowed' from any golf course or building site) I've got the wherewithal to make up to 30 durable non bouncy sling missiles. As I'm using 3 ballons per ball I'll only get 20.
However that still puts them at  a grand total of 13.25 pence per ball (15 euro cents, or 20 us cents). Which is about as affordable as you can get.

Dug out the bucket of sand in the workshop, chopped the end off the funnel to make it a better size for shoving sand down. Used the top of a 50 stack cd spindle as a mixing bowl, bottle of water and a stick for mixing.

And while I ran antivirus on a laptop I made some rusty balls

One pack of the balloons had an england shield on - which looks pretty cool.

I used 3 balloons per ball. Tried the rubber glue - but it stinks and actually partially dissolves the rubber which made it a real pain. So only tried that twice lol

I didn't weigh the sand and I didn't make them as big as jaegoors.

The small white ones are about 100 gms.
The slightly larger red one is nearer 150.

You can see they are the same size as the sand filled squash ball - but approx 50% heavier (wet sand versus dry sand).
I've stuck the big pill roller produced concrete missile in there as well for a proper comparison.

Very easy and cheap to make. You could knock out 20 or 30 while watching telly of an evening - just make them over a bucket or you'll get wet sand everywhere  
Haven't slung one yet - but they do feel like great ammo    

the idea wouldn't be to sell them - but everyone in as slinging club could buy a kit and then make their own ammo.

You get the best of all worlds. everyone using the same ammo, but at the weight they prefer and everyone makes their own

Awesome ! Gotta try this as soon as I move into my new house.

One question though : you said you tried to glue the open end of the balloon then stopped... but didn't say how you managed to seal this end on the next balloons ?
Do you keep that end open and slip the sand-in-first-balloon "head down" ? Then flip it upside down again in the 3rd balloon ?
Is this sufficient to keep the sand well in place in 1st balloon ?
Or do you leave a little more length to the 1st balloon's "tail" and fold it on the side ? (this seems nice in theory)
Or do you tie a knot ? (guess it would make an ugly bump)

Three balloons are not enough. These must be a minimum 10 balloons for a ball. I stick together the last three balloons. Thus the appearance becomes more long-lasting.
Wet sand has not proved itself. The best one one takes a little silicone spray. Not a lot of it. Thus the sand glides in the ball better.
Forces are better absorbed.
My balls exist of 15 balloons.
A balloon costs here in the wholesale 1.5 cents

A balloon is very thin. Therefore, the external balloon breaks fast.
If one sticks together three balloons it holds much longer.
Unfortunately, there are no balloons them are thicker.

if one has a tear outside one simply pulls a new balloon about that.

Many balloons press together then sand very much. One cannot press him with the hand. Only with the impact the ball goes out of shape. If he is too soft, he can already go out of shape with the shot.

http://www.youtube.com/watch?v=tBl8PuVvT7c&feature=results_video&lr=1&ob=0

   I personally use 6 Balloons when I make em.  This seems (to me) to be the best balance between cost and durability.  The only complaint that I have with these is that sometimes, if I forget to throw the rustys in the proper orientation (meaning with the open end of the last balloon to the rear of the projectile), they can shed a few (and in at least one case, all) layers of balloons as they travel.  Sometimes this happens when they hit things just right too. 
   
   I've been thinking lately that I might try adding a small patch of duct tape to the opening of the last balloon to seal it, but I that may have it's own issues.  Maybe Super-Glue?

Super glue will probably eat a hole in the balloon