>>63987919 (OP)
It is a suppressed MAC-10 with a scope somehow attached to the suppressor (AKA "silencer", but suppressor is the correct technical term).
John Carpenter tended to follow rule of cool, so he would do things like this in his films that don't necessarily make sense. But taking the question at face value, a suppressed MAC-10 is not a bad weapon for the task that Snake was sent to do.
Snake's job was to infiltrate a prison colony, extract a VIP (or at least his briefcase) and get out of there. He wasn't supposed to fight every convict, or really engage at long distance. Get in, get out. The MAC-10 was designed specifically for this sort of application. It is compact, decent magazine capacity and 45 is naturally subsonic so it suppresses well without compromising performance. As long as you don't have to fight a huge crowd of enemies at the same time, it is a great weapon for moving through an urban environment and quietly neutralizing a few obstacles in your way.
The scope doesn't make any sense. They mounted it on the suppressor because of the moving charging handle on top of the receiver. Side charging MAC clones exist now, which allows for optics to be mounted on top of the receiver, but not at the time the film was made.
Mounting the scope on the suppressor doesn't make much sense because the suppressor is not fixed very firmly on the gun. Removing and reattaching it invites the possibility of the scope not properly aligning with the point of impact. The suppressor does need to be reasonably parallel to the bore in order to prevent baffle strikes, but this might not be exact enough for a practical scope mounting solution.
Also, the MAC is open bolt operated. There is a delay and a significant shift in the balance of the gun between the trigger being pulled and the bolt closing to fire. This makes it very difficult to pull off accurate single shots at anything past short range, and renders the entire concept of a magnified optic extremely impractical.