Use this tool after bending the top rails on your flat to scribe where to trim your top rails. All you do is put some layout die or paint on the top rails and line the jig up with the front of the receiver then scribe a line down the rail. It's a perfect way to make the initial cuts with either a dremel tool or milling machine. These are made out of 6061 aluminum on a CNC machining center so don't expect to clamp it on and use it as a guide for a dremel. After making your initial cuts with the cut-off wheel use the small sanding drum to put the radius in the corners to make it look more original.