Tennis balls at 2oz are decent-weight ammo.  AND wild shots aren't (too) dangerous.  AND you can find/reuse most of the balls you pitch.

BUT making a sling pocket that will nicely conform to then release a bulky tennis ball is a little bit of a problem.

Here's my solution, and it works great!  It's 3 pieces of heavy woven poly strap material, each 1"wide x 8"long.  Right after you cut the pieces, heat the very ends over a flame to keep them from unraveling.

Heavily coat each end of each piece with PVC pipe cement down 1".  Use several coats of cement.  This makes a 1"square section on each end that's almost solid plastic.

Gently drill (yes drill) 1/4"holes in the middle of the 1"square coated sections.  Install brass tent grommets in these holes.  String the strap pieces together on your sling cords.  The picture below shows how this is done.

The result is a broad (about 2.375") curved AND cupped pocket that holds a tennis ball just right.  The weight of the pocket gives even tennis balls a "big ammo" feel, and you will be surprised at how hard and far you can pitch this "fluffy" ammo.

AND with this you can practice "safe slinging"!   mgreenfield

Good design. 

If leather was used it would have to be heavy and stiff so the stone would not come out.

This design also has a neat look to it with the overlapping pieces.

Good job!

jeff <><

That's really nice!
Although I have no difficulty slinging "raquetballs" from my regular sling.
Johnny

Did this sling make it into the articles yet?
If not, it should.

Willeke

that is  a very cool sling.  i just use a larger-than-my-other-sling apache style sling and that handles tennis balls really well.  i like that design though, and i think that it would be nice in leather, and might even work without heavy leather.

You would not get the same efect with just folding.
One of the great things of this sling is that it adjust to your ammo, if you fold your strands are more fixed in their position.
You can use a 2 strap version of this sling too, again better if cut rather than folded.

Willeke