For those who don't play it, KF2 changed the perks a bit and firebug was given one that is supposed to increase fuel tanks by 100 percent. Except it applies to all weapons, including ones normally excluded from magazine upgrades.
The Double-barrel shotgun has 4 rounds before reloading and even things like the M79 and RPG has two rounds (RPG is bugged and will automatically reload unless you cancel or spam click during zedtime).