[DUST] Please help me identify a shotgun (or grenade launcher) on a miniature

[Glitzer]

Does anyone by chance know what kind of pump action gun the Dust Leathernecks USMC Mortar gunners use? Is it just a fictional gun? The only thing I found that bears some resemblance is the China Lake Grenade Launcher (but that would be from the Vietnam War and also would be a little bit shorter in 1:48)
 

[Neonwraith]

I think it is no historical Shotgun, too big and the Barrel has the same lenght as the magazine.

[Devoted of Slaanesh]

After all it's Weird WWII so some things don't have analog in real life.