The All Terrain MegaCaliber Six (AT-M6), also known as the Gorilla Walker or First Order Heavy Assault Walker, represented a mobile heavy artillery walker deployed by the First Order throughout their conflict against the Resistance. As the most massive walker within the First Order's forces, the AT-M6 dwarfed the All Terrain Armored Transport utilized by the Galactic Empire, and was controlled by specialized First Order Walker Drivers. These drivers operated the machine's signature weapon: a single MegaCaliber Six turbolaser cannon mounted on its back, which gave the walker its name.
The First Order military employed the AT-M6, a heavily armored walker, during their war against the Resistance. It was designed to be both a siege engine and a psychological weapon. Its immense size, imposing limbs, and frightening sounds could demoralize entire armies before its weapons even came into range. The AT-M6 functioned primarily as a siege platform, with its entire structure designed around housing the MegaCaliber Six turbolaser cannon. This powerful turbolaser could penetrate planetary shields, ending sieges with a single, devastating shot. First Order engineers successfully integrated the massive cannon by strengthening the front legs, providing the walker with stability and giving it the appearance of a colossal ape-like predator. To power its main weapon, the walker included a dedicated power generator along with a number of auxiliary fuel cells positioned on the walker's back, behind the cannon compartment, in order to shorten the weapon's recharge time.
The AT-M6 was encased in advanced Ultra-dense matrixed composite armor and powered by a fusion reactor. In addition to the MegaCaliber cannon, it featured two forward-facing, linked heavy laser cannons, two cheek-mounted medium laser cannons intended for anti-ship purposes, and two sets of serrated cable cutters on the front legs to prevent attempts to topple the top-heavy walker. The walker also possessed a targeting computer that processed data from two sets of cockpit-mounted targeting sensor arrays to ensure accurate targeting.
In 34 ABY, these walkers were sent to the planet Vardos in order to suppress their former allies following the death of Gleb.
Not long after the battle on Vardos, and following the death of Supreme Leader Snoke and the rise of Kylo Ren to leadership of the First Order, a group of these walkers spearheaded the assault in a Veers Formation against the Resistance base on the planet Crait. They were accompanied by a pair of First Order All Terrain Armored Transports and Ren's Upsilon-class command shuttle, with numerous TIE/fo space superiority fighters and TIE/sf space superiority fighters providing aerial cover.
After most of the Resistance speeders and ground troops had been destroyed and the survivors retreated into the heavily armored safety of the base, the AT-M6s participated in the bombardment of the Force projection of Jedi Master Luke Skywalker, who had arrived to face Ren, his former apprentice. However, the laser cannons of the walkers proved ineffective against the illusion.
The initial official unveiling of the All Terrain MegaCaliber Six occurred on The Star Wars Show on August 23, 2017. It was specifically designed for the 2017 movie Star Wars: Episode VIII The Last Jedi. According to director Rian Johnson, the AT-M6 was intentionally designed to resemble a gorilla.
A scaled replica of the All Terrain MegaCaliber Six, along with other models and props, was exhibited within the Star Wars Launch Bay attraction during the Galactic Nights event on December 16, 2017. This was done as part of the promotional efforts for the then-newly released film, The Last Jedi.