From Lexicanum

An Exterminator is a small flamer that is seldom taken as a stand-alone weapon. It contains only enough fuel for a single shot without having to be reloaded, therefore it is commonly fitted to other weapons to augment the firepower of the user without encumbering them with another weapon to carry, or providing a degree of ranged firepower to those who only carry melee weaponry.

Common weapon fits include:

• Lasguns
• Boltguns (Though it could be said that all Combi-Flamers are bolters with an exterminator equipped, though many Combi-Weapons are specifically designed as two-in-one weapons, than simply weapons with strap-on attachments.)
• Shotguns
• Autoguns
• Eviscerators: (Exterminator/Eviscerator hybrids are particularly favoured by members of Redemption Cults.)

Sources

• http://www.specialist-games.com/assets/FO73NecRedemption.pdf

Flame Weapons

Imperium	Brazier of Holy Fire • Combi-Flamer • Exterminator • Flamer • Heavy flamer • Inferno cannon • Inferno gun • Incinerator
Chaos Space Marines	Combi-Flamer • Flamer • Heavy flamer
Tau	Flamer
Orks	Burna • Kombi Shoota-Skorcha • Scorcha
Eldar	Dragon Breath Flamer • Flamer