Yeah, the ease of construction of a sling is one of it's strongest points and biggest draw of its widespread use as a weapon. You should sort of have at least an inkling by now how to attach a pair of strings to an old piece of material so it can hold something like a rock at least most of the time. Sure, I have rocks occasionally fall out; it happens. VERY rarely, I have to add, but it does. Make a bigger pouch, use smaller rocks, attach the strings in the centre of the end of the pouch or use two strings connected to holes at either side of the pouch
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Pardon the bad ASCII drawing, but try that if you can't get it to centre with a single string design. They've always worked for me, but then again, so has not studying hard for exams and doing reports at the last minute. Just tie the two strings together in a knot about 3 inches above the end of the pouch and use two strings for the cords instead of one. Once you have that down, use a single string version. my first sling was a piece of curtain material with a hole poked in either end with a pair of scissors and two bootlaces tied to it and I could lob tennis balls for short distances with it, but because the material was too floppy, anything else tended to fall out. What material are you using? What are the dimensions? But by now, surrounded by how to guides and articles and en entire forum that's probably 80 pages by now just in the general section alone, you should know how to make a sling. Better lay off the brush squirrel for a while, buddy.