For every 100 base damage dealt per damage type, an extra fire stack will be added to a successful fire roll.
A gun that does 200 damage per shot will have 25% chance of starting one fire. Since it also does 200 damage, it is given another 2 fire stacks for a total of 3 fires. So, this changes Incendiary in the following ways on that 200 damage gun.
Current:
20% chance of starting 1 fire per shot (on a single damage)
Accelerated Fires Incendiary.
25% chance of starting 3 fires per shot (on a single damage)
This gives low ammo, high damage guns a much higher rate of set fires.