Matthias, my friend, you seem really very ironical and skeptic about dimpled sling projectiles
... I also like the sling as a primitive weapon but... are we going to prohibit dimples also in golf because of its artifitiality and to impose the smooth balls, that are more genuine?... or to suppress the weights and counterbalances, pulleys and triggers in modern archery and go back to primitive bow? I think that if some day the sling gets to become a modern sport, even olimpic, it will have to get a considerable modernization in its design, materials and projectiles, although for many of us that means to lose most of its attractiveness and its character of weapon. Often the incorporation of a weapon to the sport demands to exchange that character of weapon by a greater reach and spectacularity.
Any way, the fact is that the Magnus force affect clearly the spherical clay projectiles (and even on a lesser extent those of stone), with increases of range of 20 % or more according to my calculations. If further we mold the projectiles with a dimpled surface the range would be greater.
In any case I agree with you that the use of golf balls as sling projectiles is really more an entertainment than a proper sling shooting as we practice at present, with all its primitive flavor.
David, when I throw overhand or somewhat inclined, the projectile takes top-spin or spin to the left. I imagine that all depends on the position of the hand when releasing and of how you hold the sling. How is the position of your hand?
Gun, maybe to weight asimetrically the projectile is going to make a non straight flight, with oscillations besides a grater spin. It will be interesting to try.
Tint, it looks that your tennis serve style helps with spin, but I´m curious about the position of your hand when releasing, as I said David. Also the way that the pouche evolve along the snap must influence in addition, but in any case the important thing is to get a style that provides consistency in the spin.