Bullet Central is a secret stage that is accessed via tilting 10 times in a row.
This level represents a tall structure with jump pads and infinitely spawning mobs. In the spawn area there is loot which usually has at least one badge, though not always. Sometimes the badge will have to be bought. However this means that if a player can get through Bullet Central reliably then they can tilt repeatedly for badges and other loot.
Please note that as of version 1.13, tilting 10 times and going to Bullet Central level could cause the loss of accumulated speed stats (it is reset to the initial value you start with).
After completing Bullet Central, a player is moved to the Logistics stage. This can serve as a shortcut.
In the console version it has been removed because it has too many bullets.