Incendiary rounds are available for the Heavy Cannon, Auto Cannon, and Rocket Launcher. These rounds do some impact damage but also set the target and the surrounding area (in the case of the rocket, a LOT of the surrounding area) on fire, with the following results:
- A burning fire can also be used as an impromptu smoke screen.
- Especially useful during Night Missions, fires not only damage aliens and obstacles hiding in dark corners of the map, but also illuminate them for several turns.
- Brush and wood on fire will eventually be consumed, leaving a flat scorched earth tile.
- Aliens or soldiers on fire take slow damage (1 point/turn?) until they are unconscious or dead. (Q:Is this damage direct to hit points, bypassing armour?) The flames last for about three rounds. Power and Flying suits are fireproof.
- If a Zombie is killed by the explosive impact of any form of Incendiary munitions, the Chryssalid larva will be burnt up instead of hatching.
- UFO interiors and dropped items are fireproof, even if their owners aren't. Silverblade recommends combining this fact with the diagonal wall bug and an incendiary rocket to roast them in the shell.
Fires spread. A tile surrounded by flaming tiles will likely catch on fire next turn. This can make for a fairly rapid spread, up to one tile radius per turn, in ideal conditions (such as wheat fields and haylofts). However, the fires go out eventually and the nearby tiles have a chance to not catch on fire, limiting indefinite spread.
A soldier can walk through flaming tiles but try not to be left standing on or next to one by the end of the turn as that's when the fire spreading code runs. Incendiaries also have the effect of restricting alien movement as they'll tend to avoid the fire.
Regular explosives and explosive ammunition also have a chance of starting fires - particularly in the mountain maps. Copious use of explosives on thick cover help prevent the enemy from using cover or shooting back.
The XCOM game makes a brave attempt to model fire. However there are some quirks:
- If an alien is on fire or standing in flames, shooting another alien with an incendiary round will do impact damage to BOTH of them. Even if widely separated on the map. The effects on the first target accumulate with each incendiary round fired.
- Aliens in smoke will also take additional STUN damage every time incendiary damage is handed around.
- There is an upper limit to how many squares can be on fire. Once you have a good sized blaze going, for example a forest fire, further incendiary rounds will seem to fizzle as no more fire can be created. Impact damage will still be dealt.
- The damage value listed in the Ufopaedia does not determine how powerful the incendiary round deals damage. It only determines how wide an area will be blanketed with flames. Impact damage and damage over time have set damage values.