It may not be terribly creative, but surely it'd be easier for Tripwire to implement this than a brand new weapon.
Just tweak/modify the Husk's fire cannon a little, such as it would use ammo or be on a short recharge timer. If it used a timer, and not ammo, the Firebug would suddenly join the Zerker in 'survivalist' mentality, meaning the firebug would no longer run out of 'ammo' for his weapon the way the Zerker lives off of the Katana.
Story-wise, you could just say The Trader found some spare Husk cannons, modified them, and is now offering them to you!...
for a nominal fee, of course.
Just tweak/modify the Husk's fire cannon a little, such as it would use ammo or be on a short recharge timer. If it used a timer, and not ammo, the Firebug would suddenly join the Zerker in 'survivalist' mentality, meaning the firebug would no longer run out of 'ammo' for his weapon the way the Zerker lives off of the Katana.
Story-wise, you could just say The Trader found some spare Husk cannons, modified them, and is now offering them to you!...
for a nominal fee, of course.