What should the designers have done - make one gun unrealistically different from the other or just not listed one of them?
I'm certainly not craving for realism, cos all the weapons are unrealistic (otherwise they would have added Armour Piercing, RAte of Recoil, Overheat...) and I don't care cos it's a game. What I do want though is the same weapon balance as found in the PHB. I think the designers had some idea to do it right, but eventually they put everything into one sloppy list (due to time pressure??).
It DOES say it becomes smaller without the stock! Read it again. And it's backed up in Ultramodern firearms with further description about that feature.
I read it again and it does NOT say anything of it becoming Medium or so. Now you read it again.
P101: "It features a collapsible stock, making it extremely compact."
Nice flavor text, but how "compact" is it?
Steyr comes witha built in optical scope, increasing it's effective range. Again, the scope is mentioned in the description.
"It features a built-in optical sight."
Again, I consider this flavor text cos there's no such thing as an "optical sight" in the book. If they meant to give it away for free, they would have written "scope, see pX" etc.
The MAC Ingram for ex also says that it can have a silencer without modification, but it doesn't mention it HAS one for free!
It SHOULD have been a lighter weapon, but the rules explicitely state it's Large ! Please look it up; I think you're guessing what it should be (and I agree) but the book sadly reads otherwise. I'll give you some more examples but so far you didn't prove me wrong.
If you want to know what I think is really balanced, please tell me your email and I'll send you a nice list.