Having an expansive world, One Piece features a variety of fighting styles. Some primarily use weapons like Zoro and Mihawk, as well as those whose power derives from a Devil Fruit like Luffy and Robin. Since being a pirate is a dangerous occupation, even non-combatants must have a means to protect themselves, which is the exact reason Nami constantly carries her Climate Baton. Similarly, Usopp has his slingshot, since his skills largely depend on his sniping skills. As their opponents get stronger, Usopp also improves his slingshot along the way.
Usopp's first weapon was the Ginga Pachinko, which translates to Galaxy Slingshot. It's no different from an unsuspecting kid's toy, but it turned lethal in Usopp's hands as he partnered it with his wit and trickery. Usopp used a variety of ammunition in the form of pellets. Some of his ammo included lead, gunpowder, fire, and even a rotten egg. Since the Ginga Pachinko couldn't deal much damage on its own, Usopp has to use it alongside his wide arsenal of weapons.
Ginga Pachinko remained as Usopp's primary weapon of choice during their travels in East Blue and the first few islands of Grand Line.
Usopp first discovered the wonders of dials, the shell-shaped technology, in Skypiea. He hoarded a ton of them, then developed his trusty slingshot. The Kabuto is named as such for having a similar appearance to a beetle's horn. While the metallic pronged design that stands as tall as Usopp adds an aesthetic flair, it also amplifies the slingshot's range, power, and accuracy. The Kabuto easily outranged a Marine rifle. Similar to its predecessor, its firepower largely relied on a variety of ammunition. The slingshot's size and shape allowed Usopp to use multiple pellets at once, which manifested as one big attack like the phoenix-shaped Fire Bird Star and the shotgun-Esque Sunflower Star.
When Kuma sent the Straw Hats to their would-be training grounds, Usopp reached the Bowin Islands. The island was filled with ready-to-eat food and the home to giant insects and carnivorous plants. Heracles trained and taught Usopp what he knows about Pop Greens, which the Straw Hat used to further his weapon. The Black Kabuto retained the Kabuto's design while regaining Ginga Pachinko's portability. Due to a certain species of Pop Green living inside Black Kabuto, the slingshot "grows up" when in contact with water. The Grown-Up Black Kabuto resembles the Kabuto, but the pad turns into a carnivorous plant that can even chomp gigantic rubble and use it as ammunition.
The more notable upgrade on Usopp's arsenal post-time skip is his bullets. Usopp utilized his Pop Green knowledge and used them in a variety of ways. Most of it targets wide-range areas like his Green Star: Rafflesia, Green Star: Devil, and Green Star: Sargasso. However, there are also more direct Pop Green attacks similar to his Green Star: Impact Wolf.
Even though Usopp's skills pale in comparison to Franky's, his creative mind is nothing to scoff at. His work on Nami's Climate Baton has saved the navigator on multiple occasions, and his slingshot has also undergone numerous noteworthy upgrades. Although his Black Kabuto can already contend with the strong forces of One Piece, Usopp may still improve it one way or another.