Fire damage and damage from fire stacks are two separate things. "Fire damage" refers to a damage type that works like explosive, piercing, etc. and is therefore done directly by the gunfire. You can find the numbers for how much it does on the main site. So for the flamethrower, for example, it fires 13 (I think) particles per second, each of which does 4 fire damage. Each particle has an x% (don't remember the number) chance of applying a stack. A single-stack fire does 8 damage (no modifiers) per second, with an additional 2 damage per extra stack.