it's practice. Pure and simple.
If you look at the wo guys who are currently at the top of the balearic target slinging tree. Jaegoor and luis, they practice A LOT.
Both have their own permament sling ranges and both use consistent ammo.
So that's one level of slinging.
And it has quite specific parameters that most of us will never get the chance to use on a long term basis.
Personally I like to sling with different sized, shape and weight missiles. It's harder to hit stuff but with sufficient practice you get more accurate with any ammo. and it has much better application in the real world.
That said - both luis and jaegoor can pretty much hit you at any distance with any ammo. And yes I do speak from experience lol
At the end of the day good technique comes from practice, not necessarily the other way round.
Consistent ammo is a lot less important than most of you believe (says the guy selling the moulds to make consistent ammo).
http://slinging.org/forum/YaBB.pl?num=1496247190/2#2At the end of the day the ammo is less significant than the sheer number of hours you put in training your muscle memory.