hey for all we know, the nail could very well be the Rettenjou, it was never explicitly mentioned otherwise, so it's still a valid speculation
think of it this way, her guns use seith bullets that allow them to detonate a point in space that her gun's barrel is pointing at, even if there are surfaces between her guns and the target point in space. And for moves like 2D and such, she is just detonating a point right in front of the barrel.
Not sure if this is the accurate description of how it works, but it sorta explains more or less until we have an actual full explanation