I've got to agree that Tennis Balls are the way to go for ammunition for all the reasons already stated:
Non-Lethality, Range Limitation, availability, and cost.
They can't be beat for those elements.
Now, as for a standard, one of the things to look at is regular participatory events, which requires a group, of course, but once you have that together, and let people get a few really messy sling wars out of their system on introduction (just use dodge-ball rules), you set up a target or three and get people shooting at it.
After that, you move to competitions at the local level.
Then, we need to start looking at two other types of shoot that combine a lot of what's been discussed:
Speed and target variations.
For speed, I'd say one minute per person with unlimited rounds, Score and throw for 20 meters.
For Target Variations, take a look at These, 2nd Class Figure Targets from British Musketry in 1914.
https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwjr0c6LzuPkAhUKSN8KH... The elementary target used a 12 inch ring in the center. By using the elementary targets, and scoring with a 1-2-3-4 (board, magpie, inner, bull) score, this could work nicely at 20 meters, and reward precision more. Further, it wouldn't require any separate target size or positioning. How to mark the hits would be an issue, but I'm pretty sure some chalk-dust on tennis balls would work (I haven't tried it though. If someone has, confirmation would be wonderful.)
Then, club-based team slinging is a great idea too, using standard targets. Makes the game more social, and doesn't need any extra gear.
Start mixing these in, and you'll get some good variety!