Turns out making cameras that look like rifles (or grenade launchers, in this case) was popular back in the day. Take for instance this incredibly hard-to-find Konishoruko Rokuohsha Type 89 "machine gun" variant, which found its way onto eBay for the princely sum of $US4500 ($5549).
According to International Military Antiques, the Type 89 was used to "train gunners on the famed Zero fighter plane":
A multi-purpose training device, this "gun" is effectively a camera mounted in an oversized housing, suitable for installation in a wing mount or waist-gun emplacement. When the trigger is pulled, the camera runs, with footage being taken of what the gun is aimed at, as well as an optional stopwatch; after landing, the film can be analysed by the trainers for proper lead and burst timing.
At a little over 73cm long, it would certainly feel like you were holding a rifle... or a fighter-mounted machine gun.
While it'd look amazing in anyone's pool room, $5549 is a lot to ask. But, if you're really, really into your WWII memorabilia, it's probably worth it.