The flamer is by far the most effective big gun for close-quarters action, as it doesn't have as great a warm-up as the minigun or risk-to-self as the missile launcher or Fat Man. The flamer and its ammunition are fairly common, and can be found on any Mister Gutsy (and even Mister Handy) and in many stores.
Big guns, such as the minigun and the flamer, are still pretty effective in Fallout 3 at closer ranges even if the player has almost no Big Guns skill, thanks to the game's first-person shooter design. Coupled with its high damage (a tank and a half can kill a feral ghoul reaver on very hard difficulty), it is a must-have for any big guns player.
During the last phase of the "Operation: Anchorage!" you will meet an American soldier (American Inferno Unit) carrying a flamer. If you pickpocket its ammunition, return to command tent, save and reload (while in command tent) and finally return to the trenches, the American soldier should have the flamer in his inventory. The flamer will be badly damaged unfortunately and is not affected by the Pyromaniac perk.
A sneak attack critical, or a fatal critical strike, with the flamer (as with all fire based weapons) results in the target's body being vaporized, leaving only the torso intact. If you have the Bloody Mess perk the torso may also become discombobulated. It should also be noted that anything annihilated by a critical strike will also create a small firestorm explosion similar to flammable gas, posing a slight threat to anyone within a few meters of the target.
Because of the pilot light on the front end of the flamer (or any other variation of the flamer) this weapon can ignite gas leaks simply by being drawn around them, similar to the Shishkebab and heavy incinerator. Be cautious when having the flamer drawn in an area with gas leaks, especially if the gas is used for something (e.g. Gold Ribbon Grocers)
It is wielded by a respawning raider at the raid shack and the Bethesda ruins. Both raiders reappear every 72 hours, giving you an unlimited supply of flamers and fuel for it.
If you have followers with you, it may be advisable not to use the flamer around them, as there's a good chance your allies will also be set on fire during combat. This also applies to neutral NPCs, who might accidentally turn hostile toward you.
You can hear the pilot light turn on and off when you unholster and holster the weapon.