I made my daily shooter to use twin Dankung tubes because I find these are for me the most accurate and consistent, if not the most powerful.
My pocket shooter chronos in at 5.6 ftlbs with the usual 9.5mm steel ball, sufficient to puncture steel soup cans out to 8 or 9 yds and is pleasingly accurate.
Recent experience with a slingshot rifle I bought and which had a ridiculously heavy pull but which only generated 6.4 ftlbs with 9.5mm steel lead me on to re-read in detail Richard Middleton's excellent book "Practical Guide to Man Powered Weapons and Ammunition" - this is a book that anyone interested in developing slingshots really must read, I certainly found that everything I thought I'd discovered for myself was already there in the book, with calculations to prove it!
Anyway, it transpires that the limit on slingshot velocity is the speed at which latex contracts. Once a projectile is light enough to allow the latex to contract at it's maximum velocity then going to a lighter projectile won't increase velocity and therefore there will be less power generated. The reverse is also true, so if you have very heavy bands shooting a very light projectile they'll produce no more velocity and power than lighter bands because they are already contracting as fast as they can. The way to get more power is to increase the weight of the projectile.
The Barnett Black Widow is a famous commercial slingshot and it is also famous for having a very heavy pull. I've had a couple of these in the past and got rid of them because they didn't seem any more powerful than my much lighter and easier to shoot slingshot - it turns out that was because I was shooting light projectiles.
Today I picked up another Black Widow, they still have the very heavy bands they are famous for.
To the testing:
1) Shooting 9.5mm steel, 3.55 g, 54.8gn
Pocket shooter -> 5.2 ftlbs
Black widow -> 5.4 ftlbs
(as before a disappointing performance from the Black Widow, especially given the heavy pull)
2) Shooting an M10 bolt with two nuts fitted, 38g, 586gn
Pocket shooter -> 11 ftlbs
Black Widow - >20.1 ftlbs
Now we're getting somewhere! The pocket shooter isn't happy at this weight, it feels awkward and slow, but I'm sure the Black Widow has more to come. At this projectile weight the Black Widow feels smooth and there is no band slap at all.
This is with a set of Barnett red "Magnum" bands fitted, with the standard grey bands as supplied, it maxed out at 18 ftlbs - so the Magnum bands do actually make a difference.
I've ordered some 1" steel balls and some 3/4" lead balls.
The lead balls are 28g so they'll probably come in at 16 ftlbs or so, I don't know how much the steel balls weigh - I'm hoping it's more than 38g so I can push the power further.
So there you go, the Black Widow is much maligned for it's heavy pull and relatively lackluster performance (and that's what you get from the 9.5mm steel balls they provide with the slingshot) and I wonder why Barnett don't explain how to get the best from this slingshot. I also believe that the Black Widow's famously massive pouch actually makes sense when you consider shooting 1" projectiles!
Why am I bothering? Here in the UK most weapons are very tightly controlled, but not slingshots, they can be carried and used anywhere and there is no power limit. So I'm hoping to get up to 24ftlbs, which is double the UK legal limit for air rifles (another hobby of mine) and I can carry that in my pocket!
I'll keep shooting my pocket shooter probably 95% of the time, it's just so accurate, but now I LOVE the Barnett Black Widow - give one a try with heavy ammo and see what you think.