I have a rail that clamps onto my outer barrel, I believe I got it from WGC.
I can use the rail for a number of things such as mounting a flashlight, laser, or even a bi-pod. Here you can see I've mounted my L96's bi-pod onto my M4 using this mount:
You might want to use this one:
http://www.wgcshop.com/pcart/shopper...t_Mount%20Base