It's based on a real late-war design. See here: https://armourersbench.com/2018/06/09/the-7-92mm-kurz-rheinmetall-volkssturmgewehr/
Read the description:
"THIS IS A REPRODUCTION 1 OF 10 IN THE USA OF THE WW2 GERMAN LAST DITCH PROTOTYPE VG2K 8MM KURZ THAT TAKES STG-44 MAGS. BOLT ACTION WAS MADE TO THE ORIGINAL BLUEPRINT SPECS. ONLY 1-2 ORIGINAL PROTOTYPES OF THIS GUN WERE EVER KNOWN TO HAVE BEEN MADE. THIS IS FULLY FUNCTIONAL AND LOOKS AS IT WOULD HAVE IF IT WAS MADE IN 1945."
There's your answer.
The S&W Model 76 was actually a copy of the Swedish Carl Gustav SMG, not the other way around. The US Army wanted to buy Carl Gustav M45s for special forces in Vietnam but Sweden refused to sell them due to their government's opposition to the conflict. So the US government contracted S&W to make an American-made copy of the Carl Gustav.
It's a workshop-built anti-materiel rifle, which are incredibly common in Syria and Iraq. Probably chambered in .50 BMG or 12.7x108mm.
Sure, I don't see why not.
If it's a small arm designed for lethality, I'd say it qualifies. Defining a gun as exclusively something that propels a metallic projectile is a bit of an oversight since there are rare exceptions. I'm guessing the "metallic" bit was just thrown in to stop people making pages about BB guns.
I don't feel there is any need to document fictional guns. Quite simply, the biggest issue I have there is they're not real and, more times than not, fictional guns are largely based on real guns that we do have pages for. Besides, where do you draw the line with this? If Robocop's gun gets a mention, why not the M41A pulse rifle from the Aliens films, or that one gun from Blade Runner?
I think a decent approach would be to add information of this nature to the pages of real-life guns; for example, there could be a "Appearances in media" section for guns that are particularly prolific in Hollywood films, that basically briefly touches on notable trivia instead of a list that just catalogues every appearance of the said weapon in cinema history. It might be worth noting that the Model 1887 was famously used in the Terminator films, or that the S&W Model 29 was featured in Dirty Harry - in both these cases, those guns had notable cultural impacts because of these films. It isn't, on the other hand, worth noting that X obscure gun appeared in X shooter game, because to be brutally honest, most of our readers won't care. That's IMFDB's job.
In terms of concept weapons, I think they should definitely get pages if they're worth mentioning, i.e. if there's enough information about them to justify a page and also depending on how far the projects were developed. There are countless weapon concepts that basically amounted to nothing more than a sketch on a piece of paper or a single patent that never materialized into anything. Relevance is key here. People might be interested in reading about the Saritch because of its recent exposure in video games, or reading about the WSG-2000 because it's a rare H&K prototype, but less priority should be given to things we know little to nothing about and don't have a particularly distinguishable feature about them.
Another thing to take into consideration is the danger of hoaxes when it comes to concept weapons; it's pretty easy to fake this stuff. That Gatmalite thing (supposed AR-15 with a minigun barrel, turned out to be a photoshop) was a pretty good example. We had a page about that for the longest time and only got rid of it relatively recently.
Having said that, I'm not against cataloguing as much as possible, as long as it's worth writing about and is definitely real. One of the things I like about this Wiki is the attention that goes into archiving information about obscure weapons - with or without this wiki, there are hundreds of places you can read about the Uzi, but there are seldom other websites you can read about, say, the weapons of VBR-Belgium. For some guns, our pages will be the first thing Google turns up when you enter a search. I imagine a good chunk of the traffic to this site comes from the rarer stuff that nobody else writes about.
I assume the ribbed magazines either came standard with some T2 rifles or maybe ribbed magazines of this type were available on the civilian market in the late 80s. Not sure.
Apparently the T2 was exported in limited numbers to the US under the name SAR-223, so that's probably how the production company got hold of one of these rifles.
The same rifle appears on another poster for a similar no-budget action film by the same studio, albeit with a different magazine. https://images-na.ssl-images-amazon.com/images/I/91+NRwnbEaL._RI_.jpg
I believe it's a T2 rifle from Australia.