While this isn't an "unlimited ammo" cheat, knowing the weapon spawn locations helps. In Stage 1 (City in the Sea), for example, you can find a Shotgun inside a burning car. If you avoid taking damage, you can carry that shotgun all the way to the mid-boss.
However, the term is not a myth. It refers to three distinct methods used by the modding community: Cadillacs And Dinosaurs Unlimited Gun Cheat Android