Zero Buster

The Zero Buster (ゼロバスター) and Z-Buster (ゼットバスター) are Zero's main long range weapons in the Mega Man X series.

Zero Buster
In the first three games, Zero uses the Zero Buster, a chargeable arm cannon embedded in Zero's arm(s) that is more powerful than the X-Buster. In Mega Man X, it is able to damage Vile's Ride Armor while the X-Buster couldn't. If X does not obtain the Arm Parts from the First Armor, before defeating Vile in Sigma's hideout, Zero will give his arm cannon to X.

When Zero is revived in Mega Man X2, the design of his buster changed alongside him. Zero uses his buster in his fight against X if they battle. When X is at an intermediate distance, Zero fires a small charged shot. If X is at a further distance, Zero will fire two large charged shots one after the other (one from each arm) followed by a shockwave from his beam saber. This triple attack is used by Zero to destroy his copy if X recovers all of Zero's body parts from the X-Hunters.

In Mega Man X3, the Zero Buster can be charged to use the Hyper Zero Blaster (ハイパーゼロブラスター) attack. Zero can fire two charged shots, one after the other (one from each arm); however, they do not combine. The Zero Buster has four charge levels:

When Zero became a fully playable character in Mega Man X4, his buster was completely removed due to game balancing.

Z-Buster
The Z-Buster is introduced in Mega Man X5; in this game it is only usable if the game was started with Zero, since he loses it in his battle against Sigma Head if starting with X. However, the usage of this buster was rather unfavorable since its power was stripped down drastically (in comparison to Zero Buster in the previous games). It can only be used on the ground and Zero is unable to move while firing it, because he has to "charge" the attack. He can only attack with his Lv.1 charged shot. His Lv.2 charged shots only appear during his fight as a boss.

When Zero was resurrected in Mega Man X6, the Z-Buster received new abilities. The firing frequency of the Z-Buster was vastly increased (yet still bound to ground-only usage) and its power was increased as well, making it rather useful against most enemies and bosses. After Mega Man X6, the Z-Buster was never used again in the series.

Other media
In the Rockman X manga series, Zero's buster is his primary weapon until his death in the end of Rockman X arc. When he is revived at the end of Rockman X2 arc, he mainly uses his beam saber, but at times still uses his buster. After Sigma's defeat in the Rockman X3 manga, Zero's buster malfunctions during a mission and becomes unusable; later it was destroyed when Zero used Earth Crush in another mission.

The Zero Buster is also used by Zero in Marvel vs. Capcom 3. It is very useful in terms of extending combos, to cancel into other special attacks and to put pressure to the opponent from afar. It becomes more faster when it is fully charged and can pierce the opponent for 3 hits. The colors of his charged shots are based on previous shots done in the past games: Lv1. is based on his shots from  X 5 and X6 ; Lv.2 is based on his shots from X2 and X3 and his Lv.3 is based on his shot from X2, since it is notably the most yellow.

Trivia

 * Zero's original buster has the same appearance as the Arm Parts from X's First Armor.
 * In Zero's list of attacks from Mega Man X2, in Compendium of Rockman X, the Zero Buster is named Z-Buster.
 * In the Mega Man Zero series, the Z-Buster was replaced by the Buster Shot gun, which Zero took from the deceased Milan to protect Ciel. The Buster Shot was mistranslated as "Z-Buster" in the first Mega Man Zero game, a mistake that was fixed in Mega Man Zero Collection.