These two files both show the same thing. The MPG file is much larger and will take a while to load with a modem, but is better in picture quality. Some computers may have trouble playing MOV files.
MPG file (2159K)
MOV file (194K)
Shows you how long it takes to load a sling. The stone appears to leave the sling slightly before the highest point.
Again, two files showing the same thing, but different sized and formatted files.
MPG file (1335K)
MOV file (124K)
Note that my arm doesn't quite go vertical, but a little slanted off to the right, for a stronger cast.
I'd recommend going to an empty stony beach somewhere, and slinging into the sea. This is for 1. A good supply of stones, 2. You can see where they are landing (the splashes), and 3. Safety - yours and everyone else's. I was once slinging across a river, and my stone hit a branch of an overhanging tree, bounced, and nearly hit a boy in a canoe. Oops.
Really good slingers start young. You will probably never get seriously good. I have seen film of African men keeping birds and other pests off harvested crops by piling the crops in a huge ring, and sitting on a raised platform in the centre of the ring. They stood to use the sling, and used it above the head, rotating it sideways before release. BUT they were putting next to no power in the shot - just enough to scare away the birds, so I wouldn't use this as proof of any technique used in war by the ancients. Palestinian rioters in Israel using slings used them over-arm, I saw on the news. Debates I have read and contributed to on the net have all favoured over-arm technique.
I came across an article which dealt with lead sling bullets, being tested by "expert" modern-day Balearic slingers. Some of these competed in the Barcelona Olympics. The tests used two lengths of string, which the article simply calls “long” and “short”. The long string achieved a better range: 15 metres more, or 20 more with an expert slinger. Stones did not go nearly as far as lead bullets. They have much greater surface area and much greater drag, for the same weight. 80 to 90 metres seems to be the furthest anyone can get a stone. With lead bullets slung at a concrete wall 120 metres away, the sling is said to make an impressive “impression” and make a lot of noise. The average ranges achieved for lead bullets were 112m for bullets between 20 and 40g weight, 121.8m for those 40-60g, 132m 60-80g, and 142.6m for those 80-90g. This is an increase of 10m in reach for every additional 20g. The most commonly used weights were from 60g to 80g. There was a limit of around 100g depending on the strength and arm length of the slinger. I do not find these figures very impressive.
My recent trials on Newcastle Town Moor were far from extensive, as I only had a few stones, and even with large stones picked partly for their whiteness, I usually lost them in the rough grass and the vastness of the Moor. A mediocre shot with a reasonably good pebble went about 84 yards from my short sling. With my longer sling, I was getting over 120 yards with a poor stone. I am sure that with practice I could get much better range with the long sling. I'm told that ancient Balearic slingers would often carry two slings, the unused one worn wrapped around the body or head, one short for short range use, and the other long for longer range. This now makes perfect sense to me. The shorter sling is quicker to use, more easy to use on the move, and more accurate (at least, in my hands).
Attempts have been made by members of the Yahoo slinging newsgroup (yes, there is one!) to determine the velocity of sling stones. One guy tried using a traffic speed indicator by a roadside, and found he got 44mph for a thrown stone, and no reliable reading for a sling bullet, which he took to mean that it went faster than 99mph (the limit of the machine's capability). Another guy went to a Royal Ordnance firing range and clocked a fairly consistent 30.5 mps (68mph), but he was not an expert slinger, and could only manage ranges of 150m or so. Everyone I have known report on using this sort of technology has reported trouble with it. It is never set up for slings. For some, you have to shoot the projectile past a tiny window without hitting and damaging the expensive equipment, which is easy with a modern rifle, but not so with a sling.
Recently, another sling enthusiast on the net quoted the following: "World record 2.25oz lead weight 477 meters!! (US), 8oz stone, 375 yards (British)". This is more like the ranges quoted for ancient warriors. The American record for the lead bullet used a bullet of typical ancient weight, but the British stone record used a remarkably heavy stone which surprises me. I do not know any of the details regarding these records. It could be that they were achieved with slings or slinging techniques that would be impractical on the battlefield (such as whirling the whole body round and round several times, and using a ten-foot long string).
I was sent a video which showed modern Balearic sling users in action. They were concerned with accuracy alone, and not range. Schools had targets in the playground for children to aim at. They had a technique that was a sort of compromise between side arm and overarm. They would make a few slow swirls with the sling going diagonally forwards up and left, then round back right and down, then as the sling came around again, this would change angle such that the stone left the sling at about head height. The free end of the string ended in a long stiff tassel, and from this came the sharp crack of a sonic boom when the sling was released.
Good luck. Happy slinging.