Magic swords are more interesting for the unusual properties ascribed to them, like how Excalibur's scabbard protected the person holding it. If you want to invent properties for legendary guns, you should think a bit outside the box. "It shoots... really well" is pretty boring, even a step down from "you never have to sharpen it."
Let's try one with a bit of a darker story to it. This is a very old gun, a 16th-century wheellock muzzle-loaded pistol used to assassinate a minor Czech nobleman. If it had a name, it is now forgotten, along with that of the heathen sorcerer who helped to make it. It shows signs of heavy use, but it remains as reliable as its type could have been expected to have been when it was new. The handle has clearly been replaced many times; the current one is made of ivory.
Since its accuracy is poor and it only holds one shot, it wouldn't be of much use compared to a well-made modern firearm. However, this weapon has a history. A dark rite was used in its construction, and it is said that if, when loading the weapon, you apply just a drop of your own blood to the bullet, and speak the name of your target just as you take aim on him (or her, or it), it will hit if possible. Such a wound will be fatal without exception, though if you only grazed the target it might be due to gangrene or something.
It is also said that the weapon is cursed, and that if you invoke its ability in this way, you will soon be shot yourself, and the weapon will come into the possession of the one who killed you.