Ok.
So I have been thinking, testing, thinking some more, testing...
With materials, designs... etc.
so... so far I have only tested a few variables. Pouch shape/size/design, and cord length, and cord thickness.

What are some other ones and how do they affect slinging?
How does the power of a shot mess with your accuracy and distance?
How does the stretch of the cords affect range, accuracy, power, etc?
How does the grippyness(stability) of the pouch affect accuracy and power?
Release cord stiffness?
I want to anwser some of those questions that haven't been anwsered yet.  We know more about guns than slings, and slings have been around for almost 10 times as long.
They have been far more widespread.
BTW power- how large of a rock before losing accuracy and velocity.
Stability- how often a stone flies out in the wrong direction.

Also- how much load is on the cords during a shot with a 75 gram round ball in a 35" sling?

    I think it could take volumes to answer every question about all of the variables involved with slinging. Let's deal with one for starters.. elasticity.
   Most new slingers ask this question; "Will having more stretch in your cords give you more power?"  Though a tiny bit of elasticity in your cords won't ruin your sling, you want as little as possible. Stretchy cords will give you less, not more power and they will give you a sling that is unpredictable and potentially dangerous. 

ok... so how much stretch really is bad? For what weight stone?
I intend to do some serious testing in the next few days.

BTW how much force is on the cords during the final portion of the swing(the powerstroke ;))
man, I wish Aussie was still here. I think he knew a lot about the physics of the sling...

   ... also most of us have found that shooting with full power does cost some accuracy.  The general consensus seems to be that approximately 75% or so of one's full power will give the best accuracy with a negligible loss of power.   
   Sorry about the multiple posts.. I don't have a modify tab here. 

its ok. its more helpful this way.
so does anyone know the load on the cords at the moment of release? Is it a shock loading(which elasticity would reduce, and reduce power and grip) or is it more of a static load(slowly applied or less than 1g acceleration)?

It's most definitely a shock loading because it occurs over such a short period of time, and the acceleration is quite a bit more than 9.8 m/s².

Shock loading is defined as more than 1g acceleration.
Thanks for the info... any idea on approx how much the acceleration actually is? for a 75g sphere and a 35" sling.

wait... I just realized something. What are the average projectile speeds for a sling? 150 FPS is reasonable?
what kind of speed is the sling traveling? equel or greater to?
does anyone even know?

    Exactly how much stretch is bad is hard to say.  Generally the less stretch the better.  Personally I'd consider the elasticity of natural wool to be the max.  I wouldn't feel safe using a sling with more stretch than that. 
    I prefer longer, tapered pouches.  A modified "Banana Boat" pj is a great sling. Tape pouch slings with long, tapered pouches kick butt. 
    My favorite sling is the TS3.  It, of course, has a very thin, long pouch. People often worry about slings with thin pouches prematurely dropping projectiles.  This isn't a problem at all for me.  One of my TS3s can safely hold a projectile with a diameter  approximately three times the width of its pouch.  The trick is to make the pouch very tight so that it forms sort of a "U" shape when not in use.  I don't know exactly why this makes such a huge difference but it does.  To achieve this pull the weft hard up onto the warp on every pass when weaving the pouch. 
    For general use I like a pouch approximately seven to nine inches long.  That's just a personal preference... there's no right or wrong answer to that question.  Sometimes I'll make a sling with a smaller pouch to hurl smaller ammo.  The smaller the ammo the more you can carry.  I only use such ammo for target shooting.  And sometimes I make a big pouch for some specific purpose like a staff sling or hurling snowballs.  I don't have any pictures of my snowball slings in this new computer yet but they are huge.  You could fit a grapefruit in the biggest one. 

Do you know approx how much wool stretches in percent? 10? 15? I am guessing 20-25 b/c I have used polyester yarn to make slings... a medium- tight braid(roughly 4mm in diam, 10 1mm plies, 5 strand braided) gets 25% stretch at approx 1/3 of breaking strength.
But without knowing the rough strain load on the sling. is it near the weight of the projectile or is it much higher? I am guessing approx 3+ times the projectile weight.
And i have found that cupping the pouch tends to make the sling hold projectiles better by the following mechanism-
when the projectile rolls towards the edge(not exactly rolls, but transfers force towards the edge, which when the load becomes un-centered, and it then will fall out of the pouch) the pouch hinges where the cords attach and then the projectile will slide out to one side.
Cradle slings do not hinge nearly as much as normal slings-the functional length of the pouch is longer.

anyway- I think having more elastic cords makes it more prone to the projectile falling out.

squirrelslinger wrote on Nov 26^th, 2013 at 8:55pm:

wait... I just realized something. What are the average projectile speeds for a sling? 150 FPS is reasonable? what kind of speed is the sling traveling? equel or greater to? does anyone even know?

Depends on the throw, though we can come up with an average. Acceleration will depend on how long the last phase of throw takes, the distance between the slinger's shoulder and the projectile, and the distance between their elbow and the projectile. It gets comicated because you have rotational acceleration about 2 pivot points. 3 if the sling's trailing angle changes during the throw.

dang.
ok... so i might just use the same stuff i used to figure out how the pouch stretches.

Squirrel your a man after my own heart, but this concept is much bigger than a thread or a hundred threads for that matter. I've been trying to figure out all the questions you posted for years.