What are mechanics to getting a reaction shot in Xenonauts?

Solution 1:

Initiative System

Although some forum posts incorrectly claim that reaction shots are chance based, the Xenonauts Quickstart Guide explains that reaction fire in Xenonauts works on a deterministic initiative based system. The formula to calculate a unit's initiative is:

initiative = Reflexes x Weapon Reaction Modifier x %TU_left

Notably the same rules apply for the Aliens and your soldiers in their turn and yours. This means that soldiers carrying pistols and shotguns will also trigger fewer reaction shots!

Weapon Reaction Modifiers

• Pistols & Shotguns: 1.5
• Rifles: 1.0
• Sniper & MG: 0.5

The same modifiers apply to the later replacements and most Alien weapons. The Alien Sniper breaks the pattern: its reaction modifier is 1.0.

Example

PFC. Kuznetsova has 27 of 45 TU left and is using a rifle. Her Reflexes stat is 53. Her resulting initiative score is 53 x 1.0 x 0.6 = 31.8.
An Andron Guard (TU: 45, RFL: 60) steps into her field of view. The Andron is carrying an Alien Plasma Rifle and has taken three steps already. The Andron has 36 TU left and therefore its initiative is 60 x 1.0 x 36 / 45 = 46.7. The Andron turns 45° to face Kuznetsova (1 TU) and takes a snap shot (13 TU), reducing it to 22 TU. The Andron now has an initiative of 60 x 1.0 x 22 / 45 = 29.3 which is below Kuznetsova's 31.8. Kuznetsova gets a reaction shot, before the Andron spends its remaining TU.

Had PFC. Kuznetsova been equipped with a shotgun or pistol, her initiative would have been 53 x 1.5 x 0.6 = 47.7 and she would have gotten a reaction shot when the Andron walked into her field of vision. Vice versa, had the Andron stopped performing actions after stepping into sight, Kuznetsova would not have gotten a reaction shot at all.

Some Tactical Implications

Understanding the above mechanic provides some tactical implications.

• If multiple soldiers are in the field of view of an enemy, the enemy's reaction fire can be delayed or avoided by taking one shot at a time with each soldier before exhausting any one soldier's time units.
• When coming upon a new enemy, reaction shots can be avoided by stopping immediately, and bringing another soldier with higher initiative in to take a shot or bringing a shield up front.
• By bringing a shield up front and exhausting the shield's TU completely (e.g. by taking shots even though the success chance is tiny or crouching and standing up), all reaction shots can be exhausted and other soldiers can move in with impunity.
• Operating outside of an enemy's line of sight will never trigger reaction shots. Sneak up from behind!

Addendum: Training Reflexes

As Wipqozn explains:

Reflexes (1): A progress point is gained when a soldier performs a Reaction Fire test

And the Quickstart Guide states:

REACTION FIRE:
• If a unit has enough unspent TU to fire their weapon, they may take a Reaction Fire check when a hostile unit performs an action in the line of sight during the enemy turn

I therefore deduce that you gain a point in reflexes (since it takes one progress point to level) once per mission, whenever an enemy performs an action in their field of view while the soldier has enough TUs left to take a shot, even if the soldier does not end up taking a shot.