I was digging through the archives and came across this reference in one of David's posts (http://slinging.org/forum/YaBB.pl?num=1299262416):

"...The Hawaiian slingstone was biconical, was 2.65 inches long and weighed 4.73 ounces, a little less than a baseball. The natives shaped it by rolling it between flat stones with motion to right and left as well as back and forth..."

Has anyone tried this?  I could see that, with practice, one might be able to crank out some clay ammo fairly quickly.

Thanks for the reply, Bill.  I might have misinterpreted the reference.  I thought it referred to the working of soft damp clay to shape sling bullets. 

So, that "error" led me to try what I had in mind ... basically, using a paddle or two to help roll and shape biconical clay ammo.  After a few minutes to get the basic hang of it, it worked pretty well.  I believe with practice, one could shape a really nice biconical in 15-30 seconds.

Here are my first attempts: first attempt to post a photo here; we'll see how it goes...

Clockwise from top left the damp weights are 4, 3, 1, and 2 ounces.  They are not perfectly symmetrical; I did no final hand-shaping to touch them up.  I just wanted to see what I could do with two pieces of flat ... cardboard, in this case.  I made the larger one in about 30 seconds; the smallest one took the longest at about 60 seconds.  This is about 4x faster than I can do it solely by hand.  Again, I'll bet that with practice, these could go really fast and the symmetry would improve.

For what it's worth.

This thread http://slinging.org/forum/YaBB.pl?num=1367632620/0 seems related.

The main writer was only briefly on the forum, alas.

I hand rolled quite a few biconicals, and a truckload of clay balls.

It's quite fast, after a few dozens.
I have the clay gland rolled into rough sphere or better egg shaped first,
then, i take it between my palms, and keep my hand like praying.
Then I move my palms up and down,in opposite direction, while keeping them parallel. The tip of the bicon is formed in the V shaped groove on the lower side of the palm, just above the wrist.
By applying gentle pressure, and with some experience, i think I could roll a close to perfect biconical in about 30 seconds. (If the clay is ready for use).

I don't put lead in my clay glandes but if I want some ammo, I either have to drive 30 miles to the river, where there are some rocks or make my ammo from clay that I get from the a slough that is about 300m from my house. 

I can drive my truck right up to, which is rather important if you want to fill up several buckets at one time.  Wet clay is about the same weight as water, so two buckets are about 1/2 my body weight. I weigh about 185 pounds or about 80 kg. 

I try to fill up at least 8 to 10 buckets at a time, most gets made into pots but a lot gets used for glandes.

No cracks , seems to have dried quite well, only 1 cracked and that was the one with multiple pieces of lead , it exploded in the air.

Biconical is amazing first turn with good ammo and you can see why that shapes the best.

That makes sense. My clay around here shrinks alot. It tends to be pretty light as well. Have you tried drying the clay and pounding it to dust then heating it to burn off the organic matter?

Just wondering how they refine it to a better product.