@Curious Aardvark
"It's simply the end of the release cord going faster than the speed of sound."
It's holy true but sling cord is to short and to soft to shot like with a whip. Just try to make any crack using sling without a ball.
I think the flying out ball is pulling a string and make a crack. However, I do not insist on exactly what is the mechanism of the crack.
Slow motion video would be interesting but it's all much to fast for my 240 fps camera
"If the sling if made properly then at no point should the release cord touch the ammunition. "
I think standard design give no chance of no-contact between the cord and the flying-out ball. In the worst case (the ball is placed exactly in the middle of the pouch) the whole cord is scrubbing off the ball... However, you can make sure the friction is small. As you said, "properly made sling".
"3d printed plastic sling pouches"
Interesting, how it looks? It's always good to have waterproof sling on a beach. What a great fun when using water absorbent foam balls... last year we were playing S'clash on the small, shallow river near Warsaw.
We have to repeat it... it's so hot here lately
"I have found that my 3d printed plastic sling pouches are very smooth and don't grip tennis balls very much so you get very little rotation when using them. And the ball does fly very straight."
Yes, you are right. Long time ago when I play with my dogs only (what a beautiful times, btw
) I throw for dogs tennis balls or (better) rubber balls.
I used triangular slings in those days for one only reason - it was very easy to manufacture.
Four-cord sling is even easier to do - it was my very first sling, square shape... Of course, it quickly turned out that one string is quite unnecessary and even sometimes disturb after a shot.
It was sometimes good tangled