While the pouch might seem too small for something like a tennis ball. The long strips of leather enable it to hold quite large ammo. As well as ammo of an irregular nature. While still keeping the actual pouch itself small enough that it doesn't cause any noticeable drag or air resistance.
The cupped pouch is also suitable for multiple small missiles. Which is fun.
Despite having dozens of slings of different design. My original cap paul has been my edc sling for about 5 years. I've lost count of the release cords I've worn out but it still has the original retention cord and finger loop.
Mind you even that is showing some signs of wear
Until something truly remarkable comes along. In my opinion This is the best sling design on the planet.