I just received my copy of Richard Middleton's interesting book "Man powered bullets".
It was printed before I got into slingshots (book is from 2005), so none of my devices are mentioned :-)
But there is one thing mentioned right at the start that got me thinking: The pendulum test.
Mr. Middleton's conclusion is that man powered bullets will always be slow (in comparison), but can be very heavy.
The formula for energy is e = 0,5*mass * speed * speed. So even a .22lr bullet will show much more energy than even my strongest slingshots - because the speed is squared in the energy formula and only half of the weight counts. Yet when fired into ballistic gelatin or even into water, the slingshot ball will cause a lot more damage and travel more far.
Mr. Middleton employs a different kind of test. He uses a pendulum and shoots against the lower end of it. The more it swings, the more momentum got transferred.
And it happens that a slingshot ball will move the pendulum much more than an air gun pellet or even a .22lr. And he uses fairly weak slingshots.
Momentum, as Mr. Middleton points out, is just mass times speed.
Let us compare.
The .22lr (I am using a yellow jacket HP hispeed type now) flies at 440 m/s and the bullet weights 2 gramms.
That is 194 Joules of energy.
My strongest handhelds shoot a 36 gramm .75 lead ball at 75 m/s.
That is 101 Joules.
So one could assume that the .22 is almost twice as powerful. But let us look at the momentum side.
.22 HP HS long rifle: 0.88 Newton
.75 "slingshot": 2.7 Newton
The pendulum swings out three times more far when you hit it with a slingshot (albeit a serious one).
So not only does the slingshot ball outperform the .22lr Hispeed, it even outperforms a .38 Special - which has 319 Joules of energy, but delivers only 2.13 Newton of momentum.
The slingshot is - momentumwise - on par with the 9 mm Luger (9x19 Parabellum), which brings 2.9 Newton, and just a bit under the mighty .45 ACP (3.4 Newton).
Of course the high speed of the firearms bullets have a lot of advantages. They don't drop as fast when you shoot long distances. The have way better penetration through fabric, skin and bone. But if you look at how much momentum they transfer, slingshots can compete pretty good.
PS: I recommend the book! Worth every penny, and you will have a good time.