'Sawed-off/sawn-off' or 'combat' doesn't really give any clue as to the weapon's purpose or function. 'Double-Barrelled' is a comparable mouthful and the text would probably extend beyond the screen. Really though, more likely because it rolls off the tongue easily and you only have to read it to know that it's obviously a more powerful weapon.
Or because it quite literally is a super shotgun almost 3x the damage for only 2x the ammo? Definitely something super going on there. They didn't want Doomguy to be associated with mafiosi or something?
I always wondered why the Doom (II) and Final Doom manuals never called it with its proper name, like Lupara or Sawed-Off shotgun, rather than resorting to names like 'Super Shotgun' or even 'Combat Shotgun' in some translations.