Theory One.
Usopp upgrades his Slingshoot, with Better Dials, possibly making it able to fire one handed, for quicker shots while moving around.
Theory Two.
Usopp (With the help of Franky), uses Green Pops, and Dials to create an Armor, and switches from Long Range to Short-Mid Range.
Upgrading the "Usopp Hammer" to something like the "Usopp Cactus Hammer".