Damage Industries just released their forward grips that can be mounted on rails. You get to choose between the Knob and Grenade styles... "Constructed of 6061 Aluminum with Mil-Spec hardcoat anodize. Non-protruding clamp held with stainless steel combination head (phillips / slotted) fastener. Reduced weight - cored out internal storage area (accessible by removing grip). Made in USA."