Cube will not collide no matter what I have a working gun that shoots bullets with a material named 'bullet'. I have a cube with a collision sensor that looks for the material 'bullet'. But the bullets won't collide for some reason, and the only way I can get them to collide is to put the cubes physics to Dynamic with Ghost checked, but I don't want that because it will fall through the ground. It will not collide at all if it's not that physics type.
You answer was very uninformative and i don't quite understand your issue, so please update your question and i'll update my answer accordingly. 

I'll still attempt to help by saying why it might not collide.

If your bullet is not colliding with things, the reason may be many things, so a quick troubleshoot is to check your physics tab(1) 

One, the collision group(2), if a cube had the top left box ticked (dark grey if ticked) and the floor had it's collision mask(3) on the top left as well they will collide.

And if Ghost(4) is ticked, then the cube will fall no matter what through anything, and can't be interacted with say, the mouse click.