G&G have shared details of their M16 mount:
M16 Mount
Update your classic M-16 carry handle for the modern battlefield by adding a rail mount. The Mil-Spec See-Thru Mount easily screws on top of the carrying handle to provide space for an optic or scope. Identify enemies from afar and gain a tactical advantage with the M-16 carry handle mount. Now Available Worldwide.Find your local G&G Dealer
G&G
