|“||Take two shots against a single target in quick succession. Each shot carries a -15 penalty to Aim.||”|
– In-Game Description, XCOM: Enemy Unknown
Rapid Fire is perhaps the best double shot ability in game, because it can be used after using one move or after Run & Gun (unlike Bullet Swarm or Overdrive EW) and has no cooldown (unlike Double Tap). While the shots incur the Aim penalty, taking two shots tends to average out to higher chances to hit as long as the normal shot hit chance is above 34% and below 96%. In terms of damage, Rapid fire averages out to dealing more damage so long as the normal shot hit chance is above 30%. If the first shot results in a lethal hit, the second shot doesn't trigger, which conserves ammo.