Wolfy,
You are doing wonderfully great.
Most of your balls going forward would be good, with in a 90 degree range better, and you get most between noon and the end of lunchbreak 

22 yards for tennisballs is not bad either. You will improve with training, and with ammo that has better weight and surface. But I have been slinging little balls, about golfball size but softer, and 22 yard is about my range for those, was still after a year of slinging. Now I sling what is available whereever I sling, and I have seen that with the right stone I get a better range, still notthing to brag about......but lack of training and lack of willpower might be the source of that.

I think you are already beyond me in range, and might even be better in targeting too.

Willeke

Well I did say MOST went 12 to 1 o-clock.  I was surprised at it.  Maybe it was beginner's luck.  There's a big field across from my shop I'll get to practice in tomm, less spaces for balls to hide in too. 
Dale... omg.. I love you.. you are so brillant.  I've seen those ball-throwers.  I compared them to hei-lei(spelling?) sticks in fact, when someone mentioned seeing someone 'slinging' tennis balls for dogs.  When questioned, they said they were using the stick thing.  I however never even considered using one to PICK UP the stupid tennis balls.  My knees thank you already.  I'll have to go into town to get one(of course I could order one online from the Pet supply company.. but then I have to wait for it to arrive).
I can't wait to start throwing some serious ammo!  Tommorrow is the harder heavier balls(After another round with tennis balls) to see how it runs.
Question....... my sling 'arms' are braided silk yarn and a little stretchy.  Is this bad or good? I can change them out for non-stretchy easily if it should be more non-stretchy.
I'm going to attempt to teach myself left-handed at the same time so I don't get too hard-wired into slinging right-handed.  However the first attempt today was not promising.  In fact.. it was comical.  I also need to get some sunblock! LOL.  I sunburned my left-armed tattoo!
Wolfy

"If I have seen farther, it is because I stood on the shoulders of giants."  My wife found and bought a Chuck-It a couple of years ago; we use it for the dogs.  She can chuck a tennis ball as far as I can sling one.  And Willeke posted the picture of her Chuck-It, I just copied the link.

Your comparison of a Chuck-It to a

jai alai

basket (xistera) is interesting.  So is the Wikipedia article on the sport.  Those guys can get the ball up to 302 km (187 miles) per hour?!?  We need to get those figures to the jokers that claimed that nobody can sling a rock faster than 60 mph (96 km/hr).  A sling can put a rock into at least as high an orbit as a xistera can!

As AussieSlinger noted, "stretchy" is mostly undesirable, but a bit of stretch can be nice.  On the one hand, energy spent stretching the cords, is not available to make the rock/ball/whatever fly.  On the other hand, stretching the cords is better than stretching your muscles/ligaments/joints.  I think the basic rule-of-thumb is: if your sling feels stretchy to you, use a less stretchy material.

Of the materials with which I am familiar, nylon "paracord" has about 9% stretch: if I pull a length of cord just enough to straighten it, and measure its length, and then really haul on it, it becomes almost a tenth longer, and it returns to original length when I release the tension.  Most of my slings use nylon cords, and they work fine.

Dacron cord of similar construction (3-strand kern or core, braided mantle) has about 2% stretch.  I have a couple of slings with Dacron cords.

50-pound SpiderWire fishing line (braided Spectra [ultra-long-chain polyethylene]) has about ¹/₄% stretch; my favorite

"performance" sling

has semi-home-made cords with 7 kern strands of SpiderWire, threaded up the middle of some braided nylon mason's line (round braid, no core).  But I don't use this sling too often because my finger gets sore (all the pull gets transferred straight to the finger I have the loop on).

I do not know how stretchy silk is, but you can measure it the same way I determined nylon's stretchiness.  I would be interested to know how much it stretches, and how your cords are made (braided?).

As for what you sling, yes, tennis balls can be frustrating, especially if you are trying for distance.  They are too light and too fuzzy to punch through the air like a rock can.  But for short ranges and accuracy, they are good.  Having a dog to retrieve the ball also helps.  I've figured out how to roll the ball into the pouch with my foot, so I don't get slobbery hands...