Model Gun is usually a term for a PFC (Plug Fire Cap) firing... well, model gun. Your not wrong, quite a few of them are mostly plastic and you do have to put them together a bit like a model. They're fairly accurate in that they very closely replicate the weapon they're modeled after, even down to using similar firing mechanisms. That said, they're basically glorified cap guns. They come with brass shells that you load with special caps. You load the capped shells into the gun, much like the real thing, and pull the trigger, again much like the real thing. Difference is, there's a metal plug in the barrel that pokes into the shell and hits the cap which fires it and cycles the action, replicating the real gun, in a way that can't be made to fire real bullets.
They can be fun. I have one. Mine is a copy of a Smith and Wesson M-76.
The thing about Model guns is that, the market here in the US is almost non-existent. You usually have to import the guns (I got mine from a shop in the UK) and caps. Many of them are no longer produced and spare parts aren't around so if something breaks, your probably not going to be able to fix it. The shells are expensive, some of the guns use unique shells that are hard, or impossible to find and, because they're made of soft brass, the guns tend to eat them up. They also get thrown out with quite a bit of force and, like many small things thrown behind you, sometimes they disappear. The caps are also extremely dirty and you have to clean the shells and gun after each firing or the residue will corrode the brass and some of the metal parts inside the gun.
My M-76 holds 35 shells. You have to carefully place a cap in each shell, and it's recommended to lube up each shell with some Vaseline. Helps the gun cycle better and creates more smoke. I also would spray the barrel with WD-40, again, so it ran better and made more smoke. When I shot it, if it didn't jam, it would rip through those 35 shells in about 3 seconds. Then I would spend the next half hour cleaning up the gun and the caps.
I don't shoot mine anymore. It's charm wore off years ago and I've since moved on to real guns. Only reason I still have it is because the gun it's modeled on is equally hard to find and equally expensive.
If you have your heart set on one, just be prepared to spend a lot of time and money on it. It's much more of a novelty piece than something to whip out and show off to your buds.