[Weya]

I’m gonna say “Aliens” (with Sigourney Weaver...YUMM!).

Though I think the undermount is a grenade launcher.

[Weya]

From "Alien Vs Predator: Requiem"

[Future Snake Eater]

Yeah, it's the rifle used by the Colonial Marines in "Aliens" and all other associated stories. The undermount is a grenade launcher in the movies/stories, but in reality it's a SPAS 12 shotgun (I just looked it up).

Another "Aliens" WWII weapons:

The M56A2 Smart Gun--a converted German WWII MG42.