The basic problem with moose assassins is that a moose typically lacks opposable thumbs.Or more specifically, thumbs of any description. A moose would require a rifle with a trigger that a moose could pull, I suppose with its tongue.
Designing a weapon to enable unglates to assassinate humans would be a difficult engineering problem, and in addition, might prove unwise. a weapon a moose could use could easily end up in the hands of disaffected cattle, and empowered livestock would be a threat to our way of life and eating hamburgers.