It seems to be a sight. So far thats all I know.

[squidsix]

Got this recently because it looked interesting. It appears to be some kind of grenade launcher sight maybe? Honestly no idea. Anyone have a clue?
Thanks!

 

[ContinentalOp]

NSN Parts by U S Army Armament Research: 12287833, 12312389, 12598618, 12618791, 12913470, 12931004, 12976056, 12991254, 12991504, 12991510, 12993068, 12993069, 12996823, 5049937, 5185355, 6179458, 7581430, 7598002, 7598329, 7598607

Scroll down to "12991504"...

Clamp, Synchro

III End Item Identification: Launcher, grenade, 40 mm, M203 for M16 rifle

Hope that helps.

 

[AlHunt]

All you need is the grenade launcher to go with it.

 

[squidsix]

Well how about that. Learn something every day.
Thanks!

 

[squidsix]

So...this is what five untreated serious concussions will do to you.
I went digging through my notebooks from crew served weapons instructor, and this sight is one of the first things in there.
Wear a helmet.