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Explosives
Although Gokudera has some hand-to-hand training and is more or less proficient with a handgun, he is by and large a mid-range fighter and his expertise is in explosives. On his first appearance in the manga, he is described as "a human explosive device, who conceals dynamite all over his body." At any given time, he carries an almost unending supply of regular, apparently hand-made dynamite on his person, but he also carries around several other kinds of bombs and has several different techniques for using them. In all instances, he lights his dynamite with the aid of a cigarette and for this reason he's something of a chainsmoker.
His regular approach is to throw eight sticks of dynamite at once, pulling them out of his clothes clutched between his knucles. He subsequently is able to use techniques he calls Double Bomb and Triple Bomb, which use 16 and 24 bombs respectively. He also has a skill he calls Bomb Blitz which involves rushing past an opponent and throwing up a bunch of dynamite just as he passes them.
Many of his other tricks are fairly straightforward. He uses a depth perception trick that makes use of miniatuirized bomb as well as regular size ones; this trick is usually used against opponents who are able to defuse his bombs before they do any damage. He also drops bombs around his opponent's feet as well as throwing them into the air, and will pretend to run away and hide bombs around corners, set to blow up at the time his opponent reaches them. Additionally, he alters the wicks of his dynamite to vary the timing of the explosion.
He also has several altered types of bombs aside from regular dynamite. As noted above, he carries around several mini-bombs, which he keeps in a band around his wrist that dispenses them into his palm. Mini-bombs are used to propel himself forward when he isn't able to move fast enough under his own power as well as to propel enemies away from himself if he doesn't have time to dodge.
Additionally, Gokudera has invented what he calls rocket bombs. These specialized sticks of dynamite use a cache of stored propellant at the bomb's base to alter the dynamite's original trajectory two times in the air, making them unpredictable and harder to dodge. This overcomes Gokudera's original weakness when it comes to his attack speed, and it also precludes the need for him to have to predict the timing of the explosion, the bomb's trajectory, and the movements of his opponent. What this ultimately means is that he can use less firepower more effectively.
Box Weapons
In the Reborn universe, people have the ability to generate a flame powered by their personal resolve called a dying will flame. There are seven types of flame wavelengths, each with its own special attribute: sky, sun, rain, storm, cloud, lightning and mist. Most people have only one or two wavelengths in their bodies, and even those people who have multiple flame types are only able to use one at a time--using multiple flame types simultaneously takes years of practice.
This naturally means that not only does Gokudera have five flame types, he can use all five of them simultaneously. One of his opponents goes so far as to say that Gokudera is the only person living who can do this, which indicates just how rare this sort of thing is.
It is important to note that while dying will flames are initiated by the individual's resolve, they are fed by the individual's energy: the longer they are used, the weaker they--and their user--become. They cannot be used indefinitely and the user is already injured or sick, his store of flames will be weaker than if he was at full strength.
Dying will flames are concentrated through special rings and inserted into small boxes. The flames activate the contents of the boxes, releasing them for utlitization. There are a number of different box types: animals, weapons, batteries (storage of extra energy), storage boxes, and so on. Boxes tend to be personalized for the user and his or her fighting style, and can only be opened by someone with a matching flame type (or someone with a sky flame type--sky flames can open any box, though they can not utilize all the abilities of whatever is stored in the box). Gokudera's box (or more accurately, boxes) is the Sistema C.A.I.
Sistema Cambio Arma Instanteo (Sistema C.A.I.) means "instantaneous armament change system". The system has several components, including the Vongola Storm ring and one skull-shaped ring for each of the other flame types. There are also sixteen boxes: four storm boxes, four rain boxes, four cloud boxes, two sun boxes, and two lightning boxes. The system is referred to as a puzzle box, and its components have to be opened in a prescribed order or the system won't work.
The main component of the Sistema C.A.I. is an arm-mounted cannon that shoots bullets, rockets, and gigantic freaking missiles, all of which can be enhanced by Gokudera's in the following ways:
☠Storm: Using the storm flame's degeneration attribute, these attacks deliver very punishing blows. If the cannon is not loaded with any type of ammunition, it will release a dispersed type of storm flame that can extinguish other dying will flames.
☠Rain: Mainly used to coat other flame types. The tranquility aspect of rain flames allows them to weaken and penetrate enemy defenses.
☠Sun: The sun flame's activation attribute increases the speed of rockets shot from the cannon and gives them an unpredictable rate of acceleration.
☠Cloud: Allows a single shot fired from the cannon that separates into multiple bullets using the cloud flame's propagation attribute, creating a widespread attack that is difficult to dodge.
☠Lightning: Adding lightning flames to an attack creates what Gokudera says is his strongest type of ammunition, Flame Thunder, which manifests as a solid beam and can penetrate just about anything, due to the lightning's solidification attribute.
The base of each type of ammunition is storm flames and it can be assumed that the addition of a subsequent flame type is considered a separate type of ammunition that comes from a separate box.
There are four other components of the Sistema C.A.I. Ths first is a group of ring-shaped shields fashioned to look like bones. The shields are coated with storm and rain flames and can be used to protect from attacks as well as to trap enemies. Next is a pair of contact lenses that provide a visual display to allow Gokudera to aim with extreme accuracy, taking into account and correcting for things like gravity and magnetic fields. There is also a disc propelled by storm flames on which Gokudera can ride to move at high speed, increasing his maneuverability.
The last and possibly most important component of the Sistema C.A.I. is Uri, a bad-tempered leopard cub capable of evolving into a rather large leopard with the use of concentrated sun flames. In general Uri isn't much help in her smaller form, though she is brashly courageous. However, she does have the ability to absorb flames to add to her attack power, and in her larger form, she is significantly stronger and faster.
Vongola Box and Cambio Forma
In her Cambio Forma (altered form), Uri becomes the central part of a weapon called G. Archery, a large bow made of bones with a string made of storm flames, which he draws with his free hand. Marked with a Roman numeral I and covered in Storm flames, Uri is mounted on Gokudera's left forearm in this form. She is capable of absorbing and storing flames which are then used as arrows--she can also release them to redirect Gokudera's arm if he isn't able to turn fast enough on his own.
Gokudera is shown using two types of arrows with this weapon in the canon. The first is Tornado Flame Arrow, because Gokudera sucks at naming things a cone shaped conflagration of storm flames that can pierce almost anything. The second are Gatling Arrows which can pierce a target and then anchor themselves in the ground to hold them immobile. Considering Gokudera's creativity when it comes to his weapons, it's possible that other types of arrows exist or can be created on the fly.
Originally, Uri was just one box among the sixteen that made up the Sistema C.A.I. Eventually, she is changed into her Cambio Forma as the Leopardo di Tempesta ver. Vongola, Gokudera's Vongola box. Ultimately, this box is altered into a ring shaped like Uri's head. This ring houses the contents of the Sistema C.A.I.'s original fifteen boxes and Gokudera's four skull rings. When he activates Uri using storm flames, he is then able to release the other compenents of the Sistema C.A.I. seemingly at will.
At entranceway
Before his canon update, Gokudera did not have possession of or know how to use box weapons. He came into Wonderland with only his knowledge of explosives. Since arriving, through the course of events, he learned to light his ring flame as well as that he has multiple flame types. However, as he had no boxes, this information had little practical use to him most of the time.
In Wonderland, Gokudera has adapted his explosives to new situations and new opponents. These adaptations include the invention of rocket bombs modified into slow burning flares, flash bombs, and pressure-sensitive bombs that don't need to be lit.
After his canon update, Gokudera has the the both the Vongola Storm ring and the Leopardo di Tempesta ver. Vongola ring in his possession, which necessarily means he has the Sistema C.A.I. in its entirety at his disposal. However, because box weapon animals fall under the category of animal companions that aren't allowed, he can only access Uri in her weapon form, i.e. as part of G. Archery. He won't be able to release her in her leopard form either as a cub or an adult.