The Kabuto is a dial-powered giant slingshot.
Usopp developed this weapon sometime before arriving in Enies Lobby, using the Dials that he obtained in Skypiea. It is a green metallic contraption that looks like a cross between a staff and a slingshot. It is apparently named after the beetle of the same name due to its resemblance to the beetle's horn. It is first seen at the end of Episode 274 in the anime, but isn't given any explanation until Episode 278. Usopp revealed this weapon when he was asked by Luffy to burn down the World Government flag.
This weapon is several times more powerful than his old slingshot, able to fire faster, farther, and more accurately. This is fully demonstrated when, standing on top of the Tower of Justice, he snipes down Spandam and several Marines on the bridge of hesitation, without missing a single shot; the Marines' rifles were incapable of firing that far, let alone actually hitting him. 
The Kabuto's effectiveness is thanks to its design, its five prongs that help with its stability and range. The Breath Dial also installed in it gives it more strength. When Usopp shoots, the Dial activates and gives the ball shot from the Kabuto a rotation. These open up a whole new range of attacks for Usopp to use. If the Dial that is installed is changed, the ball shot will become even more powerful. Near the end of the events in Enies Lobby, Usopp tried to explain all of this to Zoro and Sanji but was cut short. He later explains it to Perona when he fights her.
Usopp is shown to use this weapon effectively with his comrades and old arsenal after the Enies Lobby arc, but it has not been seen since the two-year time skip, during which it was upgraded to and/or replaced by the Kuro Kabuto.
The following techniques are those he can do with this weapon. As can be seen, several of these techniques are actually upgraded versions of those he can do with his slingshot.
- Hi no Tori Boshi/Fire Bird Star (火の鳥星（ファイアーバードスター） Hi no Tori Boshi/Faiyā Bādo Sutā?, literally meaning "Fire Bird Star"): Usopp fires what can be assumed to be a very strong Kaen Boshi in the shape of a massive phoenix, it also seems to summon a massive ring of fire once it makes contact. This was first shown when he burnt the World Government flag on top of the Tower of Justice.This is called Fire Bird Star in the FUNimation dub.
- Himawari Boshi (向日葵星 Himawari Boshi?, literally meaning "Sunflower Star"): Five Kayaku Boshi are shot at the same time in a pentagon pattern. This was first seen being used against Jabra. In the Viz Manga, this is called Sunflower Star.
- Cho Kemuri Boshi (超煙星 Chō Kemuri Boshi?, literally meaning "Super Smoke Star"): Usopp shoots a pellet that spews smoke over a great distance, creating an enormous smokescreen. This was first used in order to blind some Marines that were trying to destroy the Going Merry. This is called Special Attack: Super Smoke Star in the FUNimation subs.
- Rokuren Mamushi Boshi (六連蝮星 Rokuren Mamushi Boshi?, literally meaning "Six Chamber Pit-Viper Star"): Six Kayaku Boshi are shot at the same time in a snake like pattern at an enemy. This was first seen being used against Absalom as he was taking away Nami.
- En Boshi (塩星 En Boshi?, literally meaning "Salt Star"): Knowing salt to be the weakness of the zombies of Thriller Bark, Usopp launches pellets filled with it into their mouths and can also be used to blind an opponent. This can also be shot at high speed, called Gatling Salt Star in the FUNimation dub. This was first used against some zombies that were trying to capture Zoro, Sanji and Franky.
- Ageha Ryusei (アゲハ流星 Ageha Ryūsei?, literally meaning "Swallowtail Butterfly Meteor"): Usopp shoots a high speed Kayaku Boshi the size of a golf ball with a tail of smoke. This was first being used against Perona in animation. In the Viz Manga, this is called Butterfly Meteor.
- Kamakiri Ryūsei (カマキリ流星 Kamakiri Ryūsei?, literally meaning "Praying Mantis Meteor"): Usopp shoots a high speed Kayaku Boshi in an upward arc that erupts in a green explosion. This was first used against Perona in animation. In the Viz Manga, this is called Mantis Meteor.
- Atlas Suisei (アトラス彗星 Atlas Suisei?, literally meaning "Atlas Comet"): Usopp shoots four high speed Kayaku Boshi with smoke tails which converge together and hit a target. The arcs the four shots make resemble the horns of an Atlas Beetle. In the Viz Manga and FUNimation subs, this is called Atlas Comet. This was first seen being used on a wall to reveal the room containing Perona's real body.
- Torimochi Boshi (トリモチ星 Torimochi Boshi?, literally meaning "Sticky Star"): Usopp fires a high speed pellet containing a sticky substance of sorts with a smoke tail at an enemy in order to immobilize them. In the Viz Manga, this is called Sticky Star. This was first seen being used against Perona under another name called Daibakuhatsu Boshi (大爆発星 Daibakuhatsu Boshi?, literally meaning "Huge Explosion Star") in order to fool her. The fake name in the Viz Manga is Big Bang Star.
- Gokiburi Boshi (ゴキブリ星 Gokiburi Boshi?, literally meaning "Cockroach Star"): Disguised by Usopp saying Kurobikari Boshi (黒光り星 Kurobikaru Boshi?, literally meaning "Sparkling Black Star"; Shiny Black Star in the Viz Manga), Usopp fires a gigantic pellet the size of a bowling bowl at high speed containing plastic cockroaches at an enemy. Though not real, the cockroaches deliver a psychological attack onto an enemy rather than a physical one. This was first seen being used to freak out Perona. In the anime, the roaches are depicted as also being mechanical and capable of movement. In the Viz Manga, this is called Roach Star.
- Tokuyo Abura Boshi (特用油星 Tokuyō Abura Boshi?, literally meaning "Special Oil Star"): Usopp shoots three pellets containing his "special oil" to slip up an opponent. This was first seen being used to slip Oars' hand.
- Super Size Hi no Tori Boshi (スーパーサイズ火の鳥星 Sūpā Saizu Hi no Tori Boshi?, literally meaning "Super Size Fire Bird Star"): Usopp first fires one of his Tokuyō Abura Boshi at an opponent. As it flies towards the opponent, Franky ignites it with Fresh Fire. The result is a giant version of Usopp's Fire Bird Star. This was first seen being used against Oars and Moria.
- Kaenbin, Gas, Kaki, Yamai, to Kagero Ryusei (火炎瓶, ガス, 垣, 矢枚 と 影蝋流星 Kaenbin, Gas, Kaki, Yamai, to Kagerō Ryūsei?, literally meaning "Flame Bottle, Gas, Hedge, Arrow Piece, and Blazing Meteor"): Several anime exclusive techniques that Usopp used against Perona. Usopp basically fires several shots that explode in different colors. Due to the nature of how the scene which showed the techniques was played, the techniques were fired in a way in which it is hard distinguish one from another. Apart from differently colored explosions that cannot be easily be traced to the technique they correspond to, they look pretty much the same.
- Renpatsu Kaen Boshi (連発火炎星 Renpatsu Kaen Boshi?, literally meaning "Multiple Flame Stars"): Usopp shoots several Kaen Boshi at once. This was first seen to knock out several Shiki's men, in One Piece Film: Strong World.
- Tenryu Boshi (天竜星 Tenryū Boshi?, literally meaning "Heavenly Dragon Star"): Usopp shoots a large burst of lightning that takes the form of a dragon. This attack seems to be one of the most powerful Usopp has, even more powerful than his Hi no Tori Boshi attack. This was first seen being used against Shiki, but he dodged it, however it would later trigger the lightnings for Luffy's Giant Thor Axe. In the FUNimation dub, this is called Sky Dragon Star.