Ah! Check these:

by brionic , Wednesday, August 27, 2014, 08:48 (3682 days ago) @ stonewalrus

http://www.pocketpredator.com/two.html

He makes great slingshots - comes from a riflery background, and taught himself slingshotting from that perspective.

We have a pair of his - a polymer Ranger and a G10 Sniper - I prefer the Sniper design.

Watch his videos. Once you get past the production quality, they're pretty informative. We were able to hit 30 foot shots within a few minutes by following his MOA.

Now we keep our eyes peeled for spherical marbles when we're out and about. You will, too.