I bought the foam rubber practice golf balls  and they have REALLY helped my muscle memory. They seem to be safe, easy to find and can be thrown with accuracy. They have moved my slinging along fast.

https://www.youtube.com/watch?v=8arlyqyRk_Q ;   

A quick video of dinking around in the back yard using them.

Great video man I could tell you’re having fun lol. It’s infectious and makes me want to get out there and sling. I use to sling golf with some buddies of mine using these while they used clubs. They work surprisingly well. These would also make an excellent dodgeball sling ammo as well. Totally safe for people.

i have a similar practice ball , rubber, great to practice against a concrete wall and bounce the ammo back. throwing at people is discouraged if there is a potential of injury but with a few precautions this ammo would be relatively safe. real golfballs not so much 

have you tried tennis balls ?
Better weight, and both cheaper and easier to recover

That side-shoulder technique is interesting. Not very common - but the same as roman from guam uses.

Oh yeah - cool backyard