Looks like you've got a short-barrel/Shockwave set of action bars on there, the tube on the action bars comes in two lengths. Unfortunately that's not fitting without some cutting. It's same thing with the magpul forends. If you see someone with a magpul forend on a shockwave or 14 inch barrel 590 they've probably modified it, or bought one pre-modified.
I wonder if streamlight makes a version of the racker for shockwaves and short barrels? If that's the case you might be able to return it and get the right model.