Looks like the Karasawa Mk II from AC2. I'm not gonna tag it though because it's just a cosmetic similarity. If someone thinks otherwise, feel free to tag it yourself.

Best screenshot I was easily able to find: http://ravenrepublic.net/blog/wp-content/uploads/2009/10/SLP02.jpg

The laser bayonet looks new, but especially the grip and three slashes on the longest barrel segment are pretty similar.

Huh... I guess it IS the Karasaw Mk II. Guess I'll add the AC tags then. Can someone get the artist's comment on this image? That would settle it once and for all.

The artist titled the image HOMUSAWA Mk-Ⅱ.

Artist Comment:
4のムービー公開時、銃剣シーンにうっかり「KARASAWAにMOONLIGHT着剣あるで……あるでこれ……」とぬか喜びした人は今の閲覧数だけ腹筋して下さい。

The image is also tagged KARASAWA.

Let's see QB survive a KARASAWA shot at point blank range.

Last I checked this was one of, if not the most powerful handheld weapon in the AC games. +70 to scavenging for Homerun!

Meh, it's gotten weaker and weaker in every game. It was mind-blowingly broken in the first Armoured Core. Same for the MOONLIGHT.