Slingshot
"Atta boy."
That's how Yasopp complimented Usopp as his boy hit bull's eye for the first time. His bright smile stayed with Yasopp for all his years on the sea.
Many – many – years later, while being in a shootout with his son, something he never expected to happen all these years back, Yasopp is impressed. He was impressed by Usopp and his mind-blowing skills because his son while using a slingshot, shoots just as fast and accurately as he does with his rifle.
And Yasopp is insanely proud that he taught Usopp to shoot this well with a slingshot.