The hell is this part?

[salvo]

http://i.imgur.com/yvxW7HA.jpg. It appears that it would fit on an M4. My guess is the front sight/gas block fits within that hole. UGL? Bipod mount extender?

[Combat Diver]

Doesn't look like anything for a standard M4, M14, M16, Mk11, Mk12, Mk13, Mk16, Mk17, Mk18, Mk44, Mk46, Mk47, Mk48, M2HB, M240, M240 or M3 Carl G that I'm familiar with.

[JimP]

It's a "gazinta". Gazinta something or other.....

[salvo]

http://www.globalsecurity.org/milita.../image1424.jpg - PEQ2 mount