Big aliens usually take less damage from all shotguns than described unexpectedly.

[IAFJaeger]

Big aliens includes:

asw_shieldbug
asw_mortarbug
asw_drone_uber
npc_antlionguard  // both
asw_harvester

but not includes asw_boomer.

Shotguns includes:

asw_weapon_vindicator
asw_weapon_devastator
asw_weapon_shotgun
asw_weapon_combat_rifle  // alt-fire

For example:

Default damage of Jaeger's vindicator:
    105 + 70
The damage that a Mortarbug takes from Jaeger's vindicator: 
    usually 77, sometimes 154

[WhenTanksFly]

Ricochet Rifle altfire too.

To quote myself (just added Combat Rifle altfire to the list, thanks to this post):

Vindicator has -55% damage against bigger-than-Ranger aliens except Boomers; Desert Eagle/Bulldog -50% but instakills Rangers; Pump Shotgun, Devastator, Combat Rifle altfire and Ricochet Rifle altfire -40%; Mining Laser -60% but only -50% to Uber Drone, while being extra strong against rocks and instakilling biomass; Heavy Rifle +30%; Twin Pistols deal HP% damage on successive shots to Drones and Uber Drones (except electric ones in Accident 32), enough to 2-shot regular Drones on Brutal no matter marine's damage bonus; Queen has extra resistances to many weapons.

Some of these are convars:

"rd_deagle_bigalien_dmg_scale" = "0.5" client cheat replicated - Used to scale down desert eagles damage against Shieldbug, Mortarbug, Harvester, Drone Uber, Queen "rd_heavy_rifle_bigalien_dmg_scale" = "1.3" client cheat replicated - Used to scale up heavy rifle's damage against Shieldbug, Mortarbug, Harvester, Drone Uber, Queen "rd_pistols_uber_drone_additional_damage_health_percent" = "12" game cheat - Special pistol bullets provide additional damage over heavy drones

Couldn't guess the names for the rest.

[BenLubar]

several entity types have hard-coded damage resistances in their OnTakeDamage handlers and it'd probably be good to explicitly make armor types and then document what those do somewhere in the game's UI