The Ball Slug is a spherical tank developed by the Regular Army, exclusive to Metal Slug Attack. It first made its debut on the game's event titled 'Try Line 1st', where the Slug/boss can also become a playable unit.
A spherical tank developed with the latest technology available to the Regular Army. Popular within the army for its round, lightweight form and surprising maneuverability, but it fights dirty: attacking the enemy by stomping on them or shooting out light orbs in all directions, with no blind spots. The cockpit shakes a lot, so only a few people can pilot it.
- ~Metal Slug Attack in-game description.
In Metal Slug Attack's event, this boss is fought at the last stage. The Slug will use its double cannon to attack the player, when the player's unit gets closer, it rams it down with its spherical wheels. The player must be extremely cautious as the Slug is capable of dishing out extremely powerful damage per seconds with all of its attack, so using blockers are highly recommended to deal with it.
At some time when its special attack is ready, the Slug jumps then hovers in the air to rain down energy beams, and when an attack causes it to be knocked back, it closes its shell before bouncing against the player's units three times. Both of these attacks are practically annoying, as they both block projectiles at the same time and provide the Slug with invulnerability frames, making it notorious for its exceptional stalling capabilities.
|Ball Slug MKI - The very first and original version. It is ridden by Marco.|
|Ball Slug MKII - An upgraded version of the predecessor which is also painted in color blue. It lacks long-ranged attacks in an exchange to powerful melee attacks and is much more focused on close quarters combat. It uses the MKI's counterattack as its own special attack. It is ridden by Clark.|
- Its standard form resembles the Droid BB-8 from Star Wars.
|Mechanized||SV-001 | SV-001 Type-R | Slug Copter | Slug Flyer | Slug Sub | Astro Slug | Slug Gunner | Slugnoid | LV Armor | Walking Machine | Slug Mariner | Drill Slug | M-15A Bradley | Metal Claw | Forklift | Slug Mobile | Augensterm | Protogunner | Slug Digger | Slug Trolley | Slug Armor | Slug Gigant | Golden Slug | MS-Alice | Slug Attacker | Slug Defender | Ball Slug | Slug Square | TS-001 Tandem Slug | Slug Rising|
|Animal||SV-Camel | Elephant Slug | Ostrich Slug | Donkey Slug|
|Other||Middle-Son 1986 | Middle-Son 1999 | SV-002 | Borobune Slug | Motorcycle | Parachute Drop | Drop Pack | Regular Army Helicopter | Landseek | PA33-21 Boat | RA-TTT | Rootmars Slug|