The Bullet Suit is a suit item Mario can use in one of his games, to transform into Bullet Mario. The suit has a resemblance to a Bullet Bill itself. Mario can use it to act like a Bullet Bill: flying and bashing in the air. He needs to activate this ability first by jumping into the air and then use a mechanic (e.g. the player needs to shake its Wii Remote, or presses the X Button). Bullet Mario accelerates when going in the same direction as he's already going (Bullet Mario already moves by itself when acting like a Bullet Bill). If Bullet Mario changes direction, the acceleration resets to slow, but will go faster.
When flying like a Bullet Bill, Bullet Mario could break Brick Blocks and Solid Blocks (max speed only) in it's way. However, when it hits any wall, ground or ceiling, he can't act like a Bullet Bill anymore and stops accelerating, but still is Bullet Mario. This could be dangerous if there is an abyss down there. Bullet Mario still has to watch out for enemies, like any other forms.
A Bullet Suit isn't very common. In fact, it could only be found in a few levels in most worlds. It still could be found in Minigames, though. Also, a Bullet Suit just doesn't lay on the ground easily when coming out ? Blocks. It rather flops out the Block and stands on the ground with the tail end. Then it spins around like a top. It is always a surprise how it moves or anything like that; it just could be stand on one place, but also spins randomly to places.
The Bullet Suit appears in the first Fire Star course Firesmoked Hills. It is one of the few Power-Ups that doesn't stack with others.