Let's assume that parallel universes or dimensions exist. I would like to know what are the hypothetical properties needed to make a sniper rifle (and the bullets) that can snipe a person in dimension or universe A from dimension or universe B.
Waste of Energy
The amount of energy required to connect two places from two separate universes is quite literally astronomical. Possibly the equivalent to the mass of a galaxy converted into energy at 100% efficiency. If you can create and manipulate such energies you are in quite literal terms, a god. When you can unmake galaxies why even bother with a single man, let alone something so underpowered as a gun?