You say that this prop needs to fire a projectile. I am assuming given the disposable nature of the weapon this will be fired at a person? No go.
I'm assuming that this will be an amateur production without any paperwork/production number/insurance... No go.
If its for airsoft use then you would need to get inside it. You will also need to look into the law. I don't want to crush your enthusiasm but I don't think you realise just how much shit you can get in here.
Some details of what you plan to do with it would be very informative. Given its "story needs" I am pretty convinced that it's for filming. I am very concerned about set safety with anything gun shaped weather if fire blanks or a puff of smoke. You must have a scene outline... So how are you (or your actor) using it?
That's not what I'm reading, I'm inferring that OP wants to make (and sell) collectible replicas of the film prop. I'd expect that the ostensible shootability is a gimmick and that they're never intended to be used
I wouldn't consider them realistic according to the "real firearm" clause of VCRA S38, and if he's starting from a sub 1.3J airsoft toy then they're not firearms for the purposes of the Firearms Act.
I could be wrong on all of that, but taking that interpretation, I can't see that there are any legal issues beyond those of dealing in any other non-realistic non-firearm airsoft toy, plus any intellectual property considerations from ripping off the movie.