Rokushiki/Shigan

A bullet isn't needed to pierce the human body.

Shigan is one of the six techniques called "Rokushiki".

Overview
The Shigan is a close-quarter combat technique, in which the user pushes their finger into a certain target at a very high speed, leaving a wound similar to a bullet wound.

It is a very useful technique that can connect rapidly and accurately, which works well with the Rokushiki users' impressive speed and agile techniques. However it's never been used to counter or deflect another powerful attack in any fights, which is probably because the user could easily break his finger instead of just blocking it with Tekkai. The Shigan's shape is derived from the shape of the finger, hence the hit of Shigan will have more of an impact on Luffy's rubber body (which normally reflects bullets) if it is sharper than the round bullet and resembles more modern bullets.

A sharper point is added to this technique when Lucci, Jabra and Dalmatian morph into their half-beast forms, enabling the use of their claws to fire the Shigan like an actual modern bullet, instead of simply jabbing their opponent. So far, it seems each CP9 member has their own version of Shigan, aside from Blueno, Nero, and Spandam, the latter two not even knowing how to use the technique.

In a fan question on SBS, Oda said that if you got poked in the butt with a Shigan, you'd die. In another SBS, Oda also claimed that the CP9 agents would sprain their fingers when they failed in the technique during training.

Variations

 * Shigan "Oren" (指銃「黄蓮」): Used by Lucci. This attack is simply a rapid-fire version of the normal Shigan technique that hits the enemy multiple times. In the anime, he is also seen using this technique to kill the captain of a crew who raided the kingdom 15 years ago, but did not name it as "Oren". In One Piece: Unlimited Adventure (dubbed by FUNimation), this is called Shigan: Yellow Lotus, and in the FUNimation dub and Viz Manga, this is called Finger Pistol Yellow Lotus.
 * Tobu Shigan "Bachi" (飛ぶ指銃「撥」): Used by Lucci. A projectile version of Shigan that fires sharp compressed air bullets from the fingers, similar to "Rankyaku", and are therefore able to pierce Luffy's rubber body. Unlike all other versions, this type of Shigan is launched by flicking instead of jabbing. Although in the anime, Lucci and Jabra demonstrated a variant that involves jabbing. In the Viz Manga, this is called Flying Finger Pistol Sting. The attack's name is a possible play on the term 蜂 (hachi/bachi, lit. bee), hence the projectile nature of the attack and the Viz Manga translation.
 * Shigan "Madara" (指銃「斑」): Used by Lucci. Appears as Ouren, but is performed with both hands instead of one and in half-leopard form (meaning that his fingers are clawed), this makes for the ultimate Shigan barrage. In One Piece: Unlimited Adventure (dubbed by FUNimation), this is called Shigan: Spots, and in the FUNimation dub and Viz Manga, this is called Finger Pistol Spots. In One Piece: Pirate Warriors 3, Lucci can also perform this and only performs it in his human form. The kanji for 斑 also translates to "unevenness/inconsistency".
 * Bigan (鼻銃): Used by Kaku, in his half-giraffe form. In this attack, Kaku uses his nose instead of his finger to perform the attack. As his nose is much larger in diameter than a finger, the hole it creates is also much larger than that of a normal Shigan, as well as being completely square. In One Piece: Unlimited Adventure (dubbed by FUNimation), this is called Shigan: Nose Gun, and in the FUNimation dub and Viz Manga, this is called Nose Pistol.
 * Kyoku Bigan "Kirimanjaro" (極・鼻銃「麒麟マン射櫓」): Used by Kaku in half-giraffe form. After using Kirin Hodai to completely retract his neck into his body, he can fire it out like a cannon, making his Bigan much stronger. The name Kirimanjaro is a reference to "Kilimanjaro", the tallest mountain in Africa that shares the same name. In the Viz Manga, this is called Extreme Nose Pistol Giraffe Blast.
 * Jusshigan (十指銃): Used by Jabra in his half-wolf form. In this attack, Jabra puts both of his hands together and performs Shigan with all ten of his clawed fingers at once, greatly damaging the opponent. In the Viz Manga, this is called Ten Finger Pistol.
 * Gekko Jusshigan (月光十指銃): A combination of Jusshigan and Geppo. After using Geppo to get himself into the air and subsequently avoid his foe's attack, Jabra finally uses it to launch himself towards his airborne target in a vertical drop. The added momentum and the vulnerable target makes the following Jusshigan more effective, and harder to avoid. In the Viz Manga, this is called Moonlight Ten Finger Pistol.
 * Shigan Cue (指銃 Q): Used by Kumadori. In this attack, Kumadori uses his blunt staff in the same way a finger is normally used in Shigan. The Q in the name is a reference to a pool cue, which is held in the same manner that Kumadori wields his staff. In the Viz Manga, this is called Finger Pistol Q.
 * Shishi Shigan (獅子指銃): Used by Kumadori. Rather than using his own fingers, Kumadori shapes his hair into the form of hands and repeatedly uses the fingers of said hands to rapidly jab the opponent, similar to Lucci's Shigan "Madara", but less powerful. In the Viz Manga, this is called Shishi Finger Pistol.
 * Jugon (獣厳): Used by Fukuro, this is a very strong punch which is done at the same speed as Shigan, making it even more powerful. In the  In the FUNimation dub and Viz Manga, this is called Solid Beast.
 * Jugon Ogi: "Fukuro Dataki" (獣厳「奥義」"梟叩き"): A rapid-fire version of Jugon, in which Fukuro unleashes a flurry of punches at the enemy while using Soru to move around him, in order to strike from all directions. In the anime, Fukuro creates after images of himself that circle the opponent to strike them. In the Viz Manga, this is called Solid Beast Supreme Technique Owl Strike. In the FUNimation dub, this is called the Owl Thrashing. The kanji for Fukurodataki is a play on the original kanji 袋叩き (lit. Pouch Thrasher), which is read the same way and is meant to be a term for beating up someone via a gang up.
 * Shinaru Shigan "Whip" (しなる指銃「鞭」（ウイップ）): Used by Kalifa. In this attack, Kalifa pulls back her arm as if to use a normal Shigan attack, then whips her arm out at the opponent at high speed, causing her arm to appear as if it is bending like a whip would, and hits the enemy with her finger. The whip aspect of this variation results in the added effect of the target being thrown forcibly backwards. In the Viz Manga, this is called Bendy Finger Pistol Whip.

Non-Canon Variations

 * Tobu Shigan "Mitsubachi" (飛ぶ指銃「三撥」): Used by Lucci. Lucci fires the Tobu Shigan: Bachi three times consecutively. This is called Flying Finger Pistol Triple Sting in the FUNimation dub. The kanji for "mitsubachi" is a play on 蜜蜂, which is the Japanese term for a honeybee.
 * Tobu Shigan "Hibachi" (飛ぶ指銃「火撥」): Used by Lucci. Same as Tobu Shigan: Bachi, except that the projectile is a fireball, possibly created through friction, that burns the victim upon impact. "Hibachi" is a type of Japanese grill, or a braizer (and is originally read with the kanji 火鉢). This is called Flying Finger Pistol Fire Sting in the FUNimation dub.
 * Shigan "Five" (指銃「ファイブ」): Used by Shuzo, he does Shigan with all five fingers on one hand. This was first used against Luffy. In the FUNimation sub, this is called Finger Pistol Five.
 * Shigan "Ten" (指銃「テン」): Used by Shuzo, he does Shigan with all ten fingers on both hands similar to Jabra's Jusshigan. This was first used against Luffy. In the FUNimation sub, this is called Finger Pistol Ten.

Trivia

 * The Shigan technique is most likely a reference to the Shaolin Monks' fictional technique: Shaolin Finger Jab.
 * In the 3D2Y special a government agent was seen using a Haki imbued Shigan.