In TF2, how do resistances against dual-damage-type weapons work?

Question

For weapons such as the flare gun and the Detonator (which both have a dual damage type: namely fire and bullet), how is the damage dealt by these weapons affected by bullet and fire resistance?

e.g. What would be the overall effect of a flare gun hitting a Sniper wielding the Darwin's Danger Shield (15% bullet resistance) and the Bushwacka (20% fire vulnerability)? Would they simply stack additively, meaning that the flare gun would do +5% more damage to this Sniper?

Moreover, is the damage type split evenly? e.g. would an Engineer wielding the Southern Hospitality (20% fire vulnerability) experience 10% (half of 20%, assuming half of the flare gun's damage is fire-based and the other half bullet-based) more damage from the flare gun, or would it just receive 20% more damage?

I've tried looking on the damage page on the TF2 wiki, but I can't seem to find any information about dual damage types, and whether they stack.

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If I'm not articulating myself properly, or am missing any details, please let me know.

Also, for the sake of simplicity, please ignore afterburn; I only care about the initial hit on the enemy.

Answer 1

I ran some tests in MvM by giving myself resistance upgrades as a Pyro and letting flares hit me.

• No resistance: took 30 damage
• 75% fire resistance: took 7 damage (0.25x)
• 75% bullet resistance: took 7 damage (0.25x)
• Both: took 2 damage (resisted twice, [0.25*0.25]x)
• Extra damage from crit: 60 (regardless of resistance)

So, it appears that if a single hit is of multiple damage types, anything that modifies the damage of any of its types applies multiplicatively to the entire amount of damage. Crits are an additional package of damage of only the "crit" type, so resisting the base damage won't reduce the crit damage.

Applying this logic, a Sniper with the Darwin's Danger Shield (-15% bullet) and the Bushwacka (+20% fire) will take 1.02x damage from flares, taking 30.6 damage instead of 30 (big whoop).