I was reading on another website that slung stones can usually go about 90 mph, as fast as a major league baseball pitch. This is confusing though. I thought slung stones would go much faster than a thrown baseball because they are heavy enough to gain momentum, small and aerodynamic, and have the obvious (i don't like the word obvious   but) physical advantage. Any reasons for this?

Quick answer: The folks that say a slung stone can go no faster than 60 miles/hour (or 90, or whatever) read that figure somewhere, and didn't think about it.
 
The sling has a significant mechanical advantage over a bare arm: if a baseball pitcher can zing one over the plate at 90, a slinger should be able to do better.  Much better.
 
This is borne out by Thomas, who has claimed over 500 feet distance slinging a baseball (farther than any recorded home run hit that I know of ... any baseball fanatics out there?  Give me some REAL figures).
 
I tend to believe a higher estimate that I have read, of around 200 miles/hour for a slung stone.  I can't do the math right now, but the Guinness world record for a slung stone is 1,434 feet.  What velocity would be needed to reach that distance, in air? Assume a launch angle of 30 degrees above horizontal.

Me and gard have analyzed one of Yurek's projectiles to 120 km/h. On the home site of slinging.org it stands that the slingers are availebale to get the stone up to 400km/h???? how could that happen?? ???

I believe TechStuf has recorded speeds upwards of 200 - 250 mph. He was using ball bearings and a special sling with a trigger release.
 
The 400 kph claim on the front page is my estimation of the sling's upper velocity limit.  
 
Larry Bray though he might have been able to surpass the 2000 ft mark with lead projectiles.  That would require some pretty significant "muzzle" velocity.  
 
Chris
 

We all use different units on this board, depending on where we're from and our experience/education/jobs. A very useful tool for quick conversion of units is to type the desired conversion into google.
 
Example: "60m/s in mph" returns "60 (meters / second) = 134.216178 mph"
 
Much handier than loading up a java calculator every time.
 
Matthias

Alright, Techstuff!!!  Don't break the sound barrier!
 
Stein Vegard,
 
How did you and gard figure out the 120 km/h on yurek's shot?  That's really slow!  Did you mean 120 miles/hour?  Even that would be slow for a sling projectile.
 

Tint, I believed I'd heard somewhere that the distance was 20 metres, so if that's wrong...
The stone uses 0,57 second from release to when it hits the white thing.
20 meters divided on 0,57= 35,0877193 meters/second =126,315789 km/h
But, as I said, I'm not sure the distance is right.
 
Gard