This unnamed Duke Arms Company carbine is due to appear in Grand Theft Auto VI, spotted in an official screenshot in the hands of protagonist Jason, with a Duke-branded engraving visible above its magazine. It appears to be modeled on the real-world Colt LE6920 Series rifle. It also seems to be the visual basis for two related Grand Theft Auto VI firearms, the Duke special ops carbine and the Duke assault sniper rifle, both of which echo its stock and magazine styling.
Weapon data via GTA Wiki (Fandom), CC BY-SA 3.0.