The Serenity/Cortex rules don't do a good job in handling automatic fire, so I'm using rules adapted from Cyberpunk 2020.
Three Round Burst- Gives a +1 step bonus at Close and Short, but a -1 step penalty at Medium and Long. Roll 1d3 for the number of bullets that hit. On an Exceptional success at Close and Short range, all shots hit the same location. You can aim and do a called shot, but only your first bullet is guaranteed to hit the called shot location. Basic damage is only added to the first hit.
Automatic Fire- You can fire a number of rounds up to the maximum RoF of the weapon. You can fire at multiple targets, divide the number of shots fired by the number of targets, always round down (30 rounds fired at 4 targets is 7 shots per). Each additional target is an additional action, so hosing down three targets is three attacks with a two step penalty to each. For every ten rounds fired at a target, you get a +1 step bonus at Close and Short and a -1 step penalty at Medium and Long. If you hit, every number of success is a hit, only the first hit does basic damage, additional hits do the weapon's damage only. If you need an 8 to hit and get a roll of 12, four bullets hit, up to the maximum of the number of bullets fired at that target. You cannot dodge while firing full auto, even walking is an additional action.
Suppressive Fire- This is automatic fire aimed at an area and again cannot be combined with dodging, etc. Make an Average roll to hit and declare how many rounds you are firing and how wide the area in meters is. Number of rounds fired divided by area width in meters gives the Difficulty of an Athletics/Dodge roll for anyone in the area or passing through it (30 rounds into a 2 meter area is a 15 Difficulty). If the Athletics/Dodge roll is failed, the victim takes 1d6 hits in random locations, only the first hit does basic damage based on the amount the defender failed the roll by, additional hits do the weapon's damage only. Anyone in the area, whether they are hit or not, has a -2 step penalty to any skill checks requiring concentration or focus. Characters can make a Willpower/Discipline roll to negate it (that's what the Steady Calm Asset is for).
