I want to set up a distance slinging competition to take place in the autumn sometime. 

I would like some feedback on these rules and what people think could be added/adjusted.
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Categories
1.      Slings less than 75 cm ( < 75 cm)
a.      Stones
i.      Weight: Unlimited
ii.      Weight: > 100 g
iii.      Weight: > 200 g

b.      Metals
i.      Weight: Unlimited
ii.      Weight: > 100 g
iii.      Weight: > 200 g

2.      Slings greater than 75 cm ( > 75 cm)
a.      Stones

i.      Weight: Unlimited
ii.      Weight: > 100 g
iii.      Weight: > 200 g

b.      Metals
i.      Weight: Unlimited
ii.      Weight: > 100 g
iii.      Weight: > 200 g
Equipment qualification
Measuring sling length
Slings shall be measured from the center of the pouch to the beginning of the release knot or point where the sling is gripped. For smooth corded slings, the measurement shall be from the center of the pouch to the center of the retention loop.
Measuring projectile weight
A standard kitchen scale with a resolution of 0.1 g shall be used to measure projectile weight.
Marking projectiles
All projectiles will be marked with a corresponding ‘serial’ number for reference after slinging with any available non-removable marking (etching, chiseling, scratching, permanent marker writing, etc.).

Competition Rules
Slinging
10 throws by each slinger shall be made each round. Each serial number of projectile shall be slung in numerical order from lowest to highest.
A slinging area shall be defined with a line on the ground that cannot be crossed during the throw. A movable net will be placed for safety between the slinger and officiants/viewers. The slinger must remain behind the “throwing line” during and after the throw. Stepping over the line shall result in a “scratch” for that sling projectile and the distance shall be recorded as null.  The net shall be placed to stop “slip-outs” from traveling away from the intended direction. The slinger shall attempt to stay within this protected region during the sling throw (run-ups are permitted). The slinger shall attempt to throw along a predetermined direction or “slinging line”, as the distance will be recorded along that line.
Any ‘slip-out’ of a sling projectile (early release from the sling, not in the intended direction) shall result in a ‘scratch’ and the distance will be recorded as null for the projectile.

Recording distance
The distance of the furthest slung projectile from each slinger shall be recorded using time-of-flight laser surveying equipment at the closest point of impact to the “throwing line” and perpendicular to the “slinging line.” The distance shall be recorded in meters. The distance shall be measured from the throwing line to the impact point of the projectile, measured from the point closest to the throwing line that can be identified. The measured distance of alternate throws can be requested and recorded for the slinger’s information but shall not count for the round.
The best throw of each of two rounds shall be recorded for each slinger and the furthest distance shall be declared the winner of each category.

Yes, you bring your own.
Salt flats in Nevada or Utah was what I was thinking. But anything with 500m in all directions is useable.

You have 20 shots total for each category. It depends on which category you want to enter, but yes, a possible total of 120 throws that can count.

Dang, when you put it that way...that's a lot of throwing. That might take a while if there are a lot of slingers.

Perhaps in the greater than 75cm category there should be a maximum length sling just to stop anyone whirling a piece of lead on a 3m radius.

I like all the ideas!

One thing I was warned about by the administrator of the flight archery competition was to keep it simple and not have too many categories or it gets too cumbersome.

So we should think about how we want to whittle it down. Or we can just try everything the first go-round and cut it down after we realize what doesn't work.

Perhaps instead of it being categories as such  with first second and third perhaps you could just have the competition winner for each category but at the time of throwing the numbered glande and sling length are recorded.
Everyone gets a set amount of throws. If you want to burn all your throws to win one category that’s up to you. 
Just a thought