Applejack along with Twilight, Rainbow Dash and Rarity took part on the final assault during operation: "Sands of the Hourglass", during the attack Applejack lost both of her legs as a sacrifice for the greater good.
With Sweet Apple Acres and the Apple family gone AJ joined "Sol Invictus" shortly after and ultimately, became the de facto leader of the squad. Thinking of herself as useless, it was during this time that she decided to expand her knowledge on mechatronics, physics and mathematics, she would eventually become head of engineering for the Solar Empire.
During her studies she designed multiple weapons and artillery for the army while on her free time she and Rarity built new and more powerful hind legs for herself. With her new legs Applejack naturally decided to train and ultimately become a master of hand to hand combat.
With the resources of the Solar Empire at her disposal, technology got developed and upgraded as the years came and go. While most of her designs served as inspiration for the mass produced products, she ensured that Sol Invictus kept the most advanced weaponry and technology available:
In conjunction with Rarity, she continued to offer maintenance and upgrading Fluttershy's respirator and artificial ears.
She designed and upgraded Pinkie's weapon of choice: A shotgun with a grenade launcher mechanism.
Rarity's sniper is one of a kind, the first semi-automatic rifle to have seen the battlefield.
Rainbow's near indestructible, dual-retractable blades and her body suit were also designed and built by Applejack herself
The legs she uses at the time of OP Death Harmony are the first of their kind, designed to be used in combat and endure the hazards of the battlefield.
Applejack would revolutionise the armed combat on Equestria and set herself and Rarity as a pioneers on cyborg technologies. Ultimately she would focus all her efforts on the development of the most destructive war machine Equestria has ever seen. Her prototypes and schematics based on Twilight initial designs would mark the way for the future of the Metal Gears.