These devices might indeed look very impressive, but their relative complexity nevertheless does not yield the high power levels one might come to expect; 6 mm ball bearings have far too little mass, and waste most of the power generated by the multiple tubular elastics guided around pulleys - which themselves do not seem to act like those of a compound bow or crossbow. Bolts seem to do a bit better, given their larger mass. Nothing to write home about, though - see here:
A major drawback with slingshot rifles is the fact that the rubber cools down when it is elongated for longer periods of time: rubber stores heat energy which is transformed into power i.e. is released when the pouch is released for a shot.
As a fun (relatively expensive) alternative to slingshots, why not, but if it is power output you are seeking, most sturdy slingshot frames with a strong set of flat bands drawn to at least semi-"butterfly" stance will hurl a .50 caliber or larger steel ball with significantly more oomph downrange. Tin cans beware.
In fact, Starships are the way to go if you want true magnum power - have a look at this impressive video:
Least but not last, US$ 100 also buys a reasonable air rifle in .177 caliber with at least 8 to 12 ft/lbs. Less fuss, far more efficient and reliable.
I have the 17th version of Neptune after the several frustrating days of assembly the pile of nuts bolts and pulleys that arrived in three separate boxes with no instruction whatsoever. I had a extremely awesome weapon that punches holes in everything as my poor backyard cedar fence soon learned. The bands are like the only part in the last seven months that haven't broken.
It's fairly easy to repair. And can even be easily modified to take full-size 31-inch arrows with broad head or field point sticking out the end. With that set up as deeper penetration than my 80#compound bow. The ball bearing feed will wear out your arm before you fire all forty