The 'MARS' Frontier DBL was a custom Assault Rifle used by MARS. You could not purchase this weapon alone, as it was tied to the Hero MARS. You also couldn't use it on your regular agent even if you own MARS. It features decreased damage and run speed in favor of an improvement in almost every other stat.
Similar to Premade weapons, you could not use any of the weapon's components on other weapons that you may own.