The slingshot does "hit harder" i.e. impart more momentum into the target and thus more likely to knock you down.
Intuitively this seems like it would therefore cause the most damage and for several 100 years this was the prevailing logic with muskets and cannonballs.

So much so in fact that when Charles Whitworth first introduced his rifle it was dismissed by the British army partly for having too small of a bullet. Whitworth used a smaller more stable round for its increased range and accuracy/stability (though there were also concerns about "muzzle fouling" and slower reload time).
It was believed at the time that the larger (slower) much less accurate bullets from the Enfield were more effective at actually injuring enemy soldiers, but history later demonstrated that speed and penetration can have just as much (if not more) effect on soft bodies than sheer mass and momentum.

Simply put, that large slingshot round would likely knock you to the floor in the same was as an MMA fighter landing a roundhouse square in your guts would. It might even penetrate the skin a bit and embed itself in you. What it won't do however is travel through your soft tissues at high velocity and create a large "temporary cavity" which is how most firearms do their real damage.

The 9mm etc. don't carry as much overall energy as the slingshot, but they do deliver it to a soft target much more effectively (that is to say lethally). A much more informative test would have been to fire them into ballistic clay, this would have highlighted the differences between speed, momentum and penetration much more clearly. The slingshot would leave a massive dint, the bullets would leave tunnels.

That said, the point they are making does stand to some extent. If you used that slingshot on someone that was trying to shoot you there is a good chance you'd knock them down (or at least stop them taking an aimed shot back for a few seconds). Hell you might even hospitalise them with a good shot!

It's not fair to say that the slingshot is a more "powerful" weapon but I think they did clearly demonstrate that it's a viable alternative under some circumstances. In fact for defending yourself in your own home etc. it might even be better!

Little/no risk of collateral damage (unless you miss really badly)
Very cheap
Would put most people on the floor with one good hit
No firearms licence or background checks needed
More difficult for a child to misuse (Most kids would lack the strength)
Enemy wouldn't expect it
Much less likely to kill
etc. etc.

Hell I'd get one myself if UK law wouldn't fk me over for using it.
It's illegal here to use a weapon specifically intended or kept for defense. i.e. if you grab a random object like a chair and beat up an intruder that's ok, if you have a baseball bat etc. by your bedside for expressly this purpose then it's not.
Handy then that one of my broken computer chairs happens to contain a loose 1ft long iron bar. Naturally I'd never even consider using such a thing violently, but who knows what might come to hand when faced with an intruder

(Seriously though, as broken furniture its a viable means of defence, if I kept it by my bedside as a "weapon" I'd be breaking the letter of the law by using it. Fucking stupid!)

He is starting to look a little like Harry Enfield...or is it just me?

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Ya, one day I'll be a real boy!

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*promote !

Thanks for the attempted promote

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*promote !

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Harry Enfields "chums" included Paul Whitehouse, who is infinitely more funny (check the Fast Show).

Enfield tends to go for the obvious juvenile cliche jokes, his stuff looked dated even when it was new.
His best two characters are Kevin the teenager and Wayne Slob, i guess because they are juvenile cliches.

"make yourself at homo" wasn't the pinnacle of his career!

It's from Harry Enfield and Chums

The song sung everyday on this end, kinda like a mantra.....
Always great, is discovering the old and making it new again,...(a yank discovering Harry Enfield for the first time on the sift!)

That old-timey football match skit in black n' white.....watched it 10 times at least!!!

The Harry Enfield Show is a British sketch show starring Harry Enfield and Paul Whitehouse. It was first broadcast on BBC Two in 1990 in the Thursday 9pm slot that had become the traditional time for alternative comedy.

snip -- http://en.wikipedia.org/wiki/Harry_Enfield_and_Chums