|Proto Man and his Proto Buster.|
Proto Buster, known as the Blues Buster (ブルースバスター?) in Japan, is the arm cannon in Proto Man's left arm, which was created by Dr. Wily. It's Proto Man's version of the Mega Buster, but unlike it, the Proto Buster lacks a transforming mechanism, being unable to change freely into the form of an arm like the Mega Buster can. However, Proto Man appears to be able to change his arm by other means, as he is seen with both arms in many instances, like Mega Man 7 and Mega Man: The Power Battle. The Proto Buster also consumes more energy than the Mega Buster, making it unusable for long periods of combat.
- In Mega Man: Powered Up the Proto Buster can only shoot fully charged shots, without Proto Man having to charge up, although the remake's additional content is not normally considered canon.
- In Mega Man 3 the Proto Buster shoots regular sized shots, much like Mega Man's shots.
- In Mega Man 7, Proto Man shoot regular shots once more. However, his charged shots are much more devastating than Mega Man's, requiring a short charge period for a large barrage of charged shots. He's also able to fire more quickly than Mega Man, shoot more projectiles at a time, and shoot in multiple directions while in the air.
- In Mega Man & Bass, Proto Man used a charge shot against King in the first stage. Later in the game, Proto Man used the Big Bang Strike, which used up most of his energy and destroyed King's shield.
- In Mega Man 9 and Mega Man 10, the buster is only able to shoot two shots on screen and it is the only one able to shoot charged shots, which resemble Mega Man's charged shots in Mega Man 4.
- The Proto Buster in Mega Man: The Power Battle and Mega Man 2: The Power Fighters was brought down to normal strength shots and charge shots like the Mega Buster, and in the latter, Proto Man was able to use Proto Strike, a short-ranged blast.
- In Super Adventure Rockman, Proto Man uses the Proto Buster to save Mega Man from three Sniper Joes.
- In the SNK vs. Capcom: Card Fighters series, Proto Man uses the Proto Buster to attack enemies.
Appearances in other media
Proto Man has his buster in most media featuring him, including the Ruby-Spears Mega Man animated series, Mega Man: Upon a Star, the Rockman manga series, the Mega Man Megamix series, and Rockman 4Koma Great March.
Although the Proto Buster lacks a transforming mechanism like the Mega Buster in the games, Proto Man is shown to be able to change his arm in the Mega Man animated series and in Mega Man: Upon a Star. In these two animations, Proto Man used his variable weapon system only once, using the Super Arm in The Beginning and a mechanical fan created by Dr. Light in Future Beware. Also, Mega Man copies the Proto Buster in one episode from the Ruby-Spears Mega Man show.
Proto Man's buster along with his shield appear in the Capcom game Dead Rising 2: Off The Record. It can be unlocked by completing the game with S ranking.
- Although the Proto Buster appears on Proto Man's left arm in most of his appearances, in a few instances (excluding reverse sprites) such as in Rockman Complete Works illustrations, it appears on his right arm.