Uh, but why?
Call it the
'Party Van'.They were never here, just it was possible to force the game to use them, and now it's not. I won't advertise the S-Shooty Gun - you don't like it, don't use it. There are plenty other weapons to choose from. Although 18 bullets allow engaging and killing enemies who have 60+ armor, like Stormtroopers or turned-around power armor (completely impossible with TFTD rules, but perfectly normal with UFO damage rules - if the spread is high, high volume of fire pays off much better).
Why would I roll dice against heavily armoured units with a blunt object rather than use a proper can opener like Wands of Rending, seduction, high explosives, etc? Particularly when I have unlimited gear slots, and so many effective anti-armour options?
As for everyone else, the custom shooty gun shoots them dead pretty effectively while being more friendly with smoke and in general not exposing myself to enemy retaliation; super shooty is overkill.
In general, the main problem with SSG is that against the enemies where all that extra dakka might be good, there are usually far better solutions.