How to cut a mesh outline on another plane I'm going to be printing a prop gun in two halves and in order to glue it together perfectly, i want to print an outline mold of the gun so both halves can set straight. 

I tried using the knife project tool but it doesn't cut the outline of the gun.

This is what using the tool normally is like:

It only cuts the trigger guard portion.

And this is what i would like to do:

I would like to cut only the outside of the of the mesh model onto the plane in the back, is there any way to accomplish this?

I can upload the file of the half gun if requested. 

Here is the finished product, thank you:
I assume that the half-gun has a flat face on the side it's cut.

You can extrude that face out so that it goes through the plane and then subtract the gun from the plane.

Here's an example with a half-suzanne. With a bit of imagination, it looks a bit like a gun :-)



And here's the final result