Can objects step sideways by themselves?

In my Werewolf game, the players have to assassinate someone. A rank 1 theurge wants to shoot the target with a sniper rifle.

But they want to make the bullet a talen that enters the Umbra when it’s fired, then exits just before it hits its target. Their theory is the bullet will be invisible and can pass though walls.

As this sounds like a very Glass Walker or Weaver tech thing to do, is there anything in the core rules or supplements that would allow or disallow this (like a variant of the rank 4 gift “Grasp From Beyond”)?

My instinct is to rule “no”, but I’d prefer to have concrete grounds for doing so, as I hate being an arbitrary judge.