I wonder, if a GM was fine with it, could you make a broken Sword of the Cross into a Gun of the Cross? My inspiration was the Dark Tower series, and Roland Deschain's revolvers made with Excalibur.
The Gun of the Cross would have to be able to hold the nail, so it'd work in a rifle, shotgun, maybe a revolver. A revolver would probably fit the image better.
It'd have the same abilities, just It Is What It Is would be a gun.
My idea for the backstory for one of them goes like this:
Amoracchius was taken into the West by the user, to stop some big evil (a Naagloshii, perhaps). In the course of the fight, the Naagloshii scares/tricks/hires someone to pervert the Sword, and it is destroyed. A priest or some-such saved the nail, and took it to a devout smith. He didn't know how to make a sword, but he could make a gun. Thus, the Gun of the Cross was born. It was given to a sheriff, and he fought evil with it, and he passed it on to his apprentice, and it kept getting passed on.