The 0-4377 Flamethrower fires a stream of fire when the attack key is held down. This stream can inflict a good amount of damage when it hits a target and will set the target on fire, doing even more damage. However, the stream has a limited range, making it impractical at long range.
The Flamethrower is very useful against at close-quarters. Its high rate of fire and good damage allows it to quickly destroy any enemies near the player.
The Flamethrower is also handy for destroying enemy projectiles. Putting one on a gunstack and constantly holding down the fire key lets the player destroy most projectiles heading towards them.
Putting the Flamethrower higher on a gunstack gives it more range because of how large gunstacks are angled. This can be useful for reaching out and touching enemies at near-long range.
Combining the Flamethrower with the chainsaw is a good idea, as the Flamethrower can harm both the enemy the Chainsaw is hitting and enemies behind the Chainsaw victim. The Chainsaw can also finish off any enemies that manage to get past the Flamethrower(s), which completely destroys weak to medium-strength enemies at close quarters.
The Flamethrower also works on the lower part of a gunstack. By putting it on the bottom, it can serve as a personal defense weapon against enemies that somehow manage to survive the onslaught from other enemies. Its good rate of fire and damage can rip through any weak or medium-strength enemies that manage to get close to the player.
Keep in mind that the Flamethrower tends to go through its ammo reserve quickly, so one should always be on the lookout for Napalm if they intend to use it for a decent amount of time.