Making an Impact

heading into the Pacific Ocean but... at what speed?
A discussion ensued today at work over the relative power of a sling projectile versus bullets. The only academic article I could find on the subject, "The sling, The forgotten weapon of antiquity," states that even light sling stones can have a terminal velocity of up to 128 ft/sec or 87.2 miles per hour... and now I am chomping at the bit to aim a speedometer at these little guys and see if we can get a good reading... because in the end... I don't want to know what's possible, I know want to know how fast I'm actually slinging.