Yes, but little known is the fact that David's sling had extra-long straps. And even as a boy, he was known for his long arms.
Thus, the stones he was hurling were +P loads!
And let's not forget that when David previously tested his stone loads in a ballistic medium of sheep brains and pulverized locusts, penetration was greater than normal.
Of course, this was because he was using calibrated stones made by the happy slaves of the Hotep Bonded Sandstone Premium Stones co.
Any other stone might have been filled with Goliath's hair, sheepskin clothing or thick layer of desert crust and would not have been nearly as effective.