As promised, I made a tutorial on how I've been converting tennis balls into non-lethal sling ammunition that are extremely consistent, significantly "safer" and less damaging than rocks, cement, clay or lead
http://imgur.com/a/DZKiOnfGiven that the average tennis ball is generally 55-60g and that you use approximately 50g of rice per ball, this allows you to create ammunition that is consistently in the 105-110g range quickly and easily.
Accuracy Has significantly improved when compared to tennis balls due to the increased weight of the projectile and the smoothness of the tape surface, both of which decrease air resistance and deceleration.
Using high visibility tape or patterns also allow you to see projectile flight path a lot easier and greatly helps with locating projectiles that fly off.
As far as how well they hold up: ive had a few fliers that hit gravel, corners of stairs, wood, metal, and fence, and there appears to only be minor surface damage. I'd assume that if you aren't going full strength on highly abrasive or sharp pointed surfaces, they will hold up quite well for a long period of time.
I intentionally damage a ball to see repairability and this was easily done by covering punctures/tears or rewrapping with tape and/or refilling rice as needed, so long as the structure of the tennis ball isn't compromised significantly (i.e. the ball isn't torn in two).
Note that in the beginning, the word "safer" is in quotes. While these will not cause as much damage to people or property as much as denser projectiles, they still can cause injury and may break a few things around the house if you aren't careful.
Have fun everyone. Keep Slinging.