Using ray-tracing whilst ignoring blocks with no collision

I'm adding some guns to my general war mod, and currently I am using a modified version of getMouseOver .   My problem comes when the ray-tracing method reaches tall grass, where it acts as if its hit a solid block.  I would like to know how I can make it ignore blocks without collision so that it doesn't annoy people trying to shoot a target.

Thanks for the help in advance!