From the Super Mario Wiki
Bombshell Bills are the golden relatives of the Bullet Bill. Bombshell Bills are much stronger than Bullet Bills, and are positioned so as to guard extremely important areas, such as Bowser's Castle and the Palace of Shadow. However, they are instantly killed when they attack like a regular Bullet Bill even if they miss or are Superguarded. Bombshell Bills are fired by Bombshell Bill Blasters. Bombshell Bills appear in Paper Mario and Paper Mario: The Thousand-Year Door.
Like a Bullet Bill touching a Bombshell Bill outside battle will cause Mario to get hit by the First Strike.
In Super Mario RPG: Legend of the Seven Stars, the attacks Blazer (used by Blaster and Big Bertha) and Magnum (used exclusively by Smithy in his tank form) involve the firing of a golden Bullet Bill similar in appearance to Bombshell Bills.
A gold-colored Bullet Bill appeared in Super Mario Sunshine; however it is not necessarily related to the Bombshell Bills in the Paper Mario games. This type was identical to normal Bullet Bills, except for the fact that it left eight coins when defeated instead of two, and the great amount of damage they cause.
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