Maybe a tighter fitting pouch hugs onto the projectile during release, and either slows it down, misaims it, or puts a spin on it.  A move open pouch might make for a freer release.

Chris

Well, if you're talking about leather, the supple chrome-tanned skins that we see today were pretty rare back in the day, unless it was alum tawed...

For my quick reload, which I am still experimenting with, I find my chrome tanned goatskin pouch is often folded in half by the time I slip the cord through the fingers of my left hand...

I got in some good slinging today with my new stiff pouches.  They work better than the softer pouches that cup the stone.  I did not have bullseye accuracy mind you, but much better with the stiff pouch.  The pouch does not hold the stone on release and just open up.

I still find stones in the 3 + ounce range to be best, preferably 4 or even 5 in some cases.  My favorite over all seem to be about 4 ounces.

This style seems to be working best for me at this point.  7 ounce veg tanned leather with a heavy dye and good coat of neatsfoot oil.  The pouch hold open then the stone is just hanging down.  5 or 6 ounce leather would be great also.  In fact, I would venture to say that even 3 or 4 ounce veg tanned would remain stiff enough be be good.
http://im1.shutterfly.com/procserv/47b4d833b3127cceb0764bb2a4f90000001610

This leather is available at your local Tandy Leather store or from Tandy via the internet.  They also have the dye and oil.  I am using paracord on some and #72 seine twine on some.  I don't yet notice much difference in these.

Jeff,

That is odd that you were getting as good of distance with a shot sling. I found that true for me as well but I think it must be do to my lack of practice with the long sling. Physics says that the long one will throw farther Right?