oh yeah it's not the distance - my biggst problem isn't accuracy at range it's my vision.

At 100 yards/metres I have problems seeing the bloody target 

Pete and I did an experiment where I slung a variety of ammo at varying distances and by the time I was at 70 metres my biggest problem wasn't distance or accuracy - but simply tellign where the bloody missile had gone - very difficult to compensate if you've no idea how you missed in the first place.

That said, I desperately need a new prescription. Accuracy should go way up at distance after that 

On a more historical approach I've been wondering how much eyesight would have limited an experienced slinger back before glasses - which was pretty much all of them.

Given that they'd probably develop shortsight from braiding slings anyway. 
I suspect it was a much bigger factor than anyone's realised.

I think before computers and offices,much less people had problems with short sightedness.
I'm not a doctor,but i know that our bodies adapt,even to degrading levels, in certain artificial man created situations.
Living in cities, and looking at "close quarters" the majority of the time, usually a few meters in front, is totally different than being out in the wild, where you look from at distances ranging from your feet,to the mountain 10 miles away,all your life.

I can see lead glans zipping away,up to ~ 80m  if they're painted black.
MUCH more visible than lead grey.
Stones up to ~180m with good visibility and a good background color. 
Years ago my eyesight was measured above average.

As a sidenote to this, the historian Josephus mentions how during the siege of Jerusalem the defenders could see stones lobbed at them (by siege engines) and take cover-- before the Romans decided to paint the stones in dark colours.