your weapon image much be huge
and no, you cant scale to a negative value
try manually changing the scale of you weapon sprites in an image editor, this should make it easier to reposition and scale in the editor.
a huge weapon sprite serves no real purpose, it would be scaled down in the editor anyways and is just a waste of space and memory to load.
this is my method for creating weapon sprites easily:
make a 320x180 image canvas and place your weapon sprite in the canvas where you want it to be on screen, then offset the y axis in the editor to compensate for weapon bobbing
