Immediately after the Apocalypse, and before any decent weapons could be developed, all humanity could do was seize Vulgus firearms. That's how humans ended up with this weapon's prototype. Countless knights were sacrificed to steal one Vulgus machine gun.
The gun proved difficult for practical use, however, due to its excessive size and weight. The Magisters continued their efforts and developed versions of the weapon that were an appropriate size and weight for human use.
The records and materials that resulted from their work became the foundation of countless other weapons and firearms that were later developed. It was, indeed, an "Enduring Legacy."