Question

why are neutrons more effective as nuclear bullets than protons

Answer 1

They are heavier (this means they they can carry more energy, though it isn't significantly larger when compared to that of a proton); the neutron is electrically neutral, this makes it possible to better reach the nucleus of the atoms without interacting much with the electrons (they don't lose so much energy as protons would).

Though neutrons have these properties, the fact that they are better nuclear bullets than protons depends on the medium and atoms that are involved.