When Albion first began fighting back against the Vulgus, they were lacking in many areas. The only people who could even fight in battle were the surviving knights and soldiers. Humanity only began developing weapons in earnest once the Magisters joined Albion. The Albion forces decided to form a Special Cavalry using the surviving knights to carry out high-speed mobile strike operations.
For these operations, a heavy machine gun that could be used atop a horse was first developed. The first version was much larger and heavier than the current version. Of course, mobility was not an issue when riding atop a moving horse, but on foot, its size and weight proved to be a problem. There were many times soldiers had to fully disarm and retreat. The Magisters conducted extensive research into this issue and ultimately succeeded in achieving a smaller and lighter weapon that could be used easily by anyone.
Because it was originally developed for mobility, even after its transformation, it continued to shine when used by a Descendant on the move. Since then, its appearance has not changed and it is still referred to as the "Albion Cavalry Gun."