If you mix potato flour and water you'll get a funny thing, which when held still will be like sauce, but if you knead it, it'll pretty compact.
The more water you have, the faster it'll get like sauce when not being knead.

So therefore is my idea to make this thing and have not so much water. Then, if it works, you'll be able to make a small ball of this, put it in the sling and throw, and in air it'll get like sauce and what you hit will be full of this strange mass.

It worked when I just threw it by hand. It hit the house as sauce, dripping slowly downwards the wall.

I haven't tried yet with sling, but when I do, I'll tell how it worked.

Gard

Seems to me that you can do the same thing with corn starch. If it impacts, its solid.