Guns and explosives get their targeting rolls split up, but Wizards have to make the split in the power of their spell, instead of the control of it. Weird.
That's not quite correct, what you are describing are two different mechanics. One is a zone attack, the other one is a split attack.
For a zone attack, you need to dedicate 2 shifts of power of your spell to spreading out the spell to cover the whole zone. The targeting roll stays as is. Since firearms have a fixed weapon rating, you take the -2 penalty on the targeting roll instead, when making a zone attack with one.
Example:
- A 4 shift spell with a 5 shift targeting/control roll done as a zone attack means everyone inside the zone has to defend against a 5 shift attack with weapon:2
- Covering a zone with fire from an assault rifle (weapon:3), you roll a 6, which means everyone inside the zone has to defend against a 4 shift attack against a weapon:3
A split attack is something different. Here, you take your spell and divide it into multiple pieces. You also divide your targeting roll, and assign a number of shifts to each attack. Those new numbers are what each individual target has to defend against. For firearms, you are also going to split your attack roll into multiple parts, but each individual attack gets the full weapon rating of the gun.
Example:
- A 4 shift spell with a 5 shift targeting/control roll done as a split attack against 2 targets, which results in:
target 1: 2 shift attack, weapon:2
target 2: 3 shift attack, weapon:2
- Shooting at 2 targets with an assault rifle (weapon:3), you roll a 6 on your split attack, which leads to:
target 1: 3 shift attack, weapon: 3
target 2: 3 shift attack, weapon: 3