Weapons damage card:

HEALTH AND ARMOR

Armor absorbs 80% of any damage taken. You always take a 20% hit to your health regardless of how much armor you have. This means for example that if you have 20 hp and 100 armor, 100 points of damage will still kill you.

MELEE WEAPONS

Crowbar

Body Damage: 25
Head Damage: 25
Self Harming: N/A

Stunstick

Body Damage: 40
Head Damage: 40
Self Harming: N/A

HITSCAN WEAPONS

Hitscan weapons are ones where there is no delay between when you pull the trigger and the round hitting the target. This means you should always try to line up your target perfectly each time you fire. All hitscan weapons do 3 times the damage when shot to the head.

Pistol

Body Damage: 8
Head Damage: 24
Self Harming: N/A

SMG

Body Damage: 5
Head Damage: 15
Self Harming: N/A

AR2 (AKA Combine Rifle or Overwatch Standard Issue)

Body Damage: 11
Head Damage: 33
Self Harming: N/A

Mag/357

Body Damage: 75
Head Damage: 225
Self Harming: N/A

Shotgun

Single Barrel

Body Damage: 9 - 63 (7 shots x 9 damage)
Head Damage: 27 - 189 (7 shots x 27 damage)
Self Harming: N/A

Primary fire uses one shell to release 7 shots in close formation, each worth 9 points damage. This may therefore cause 9 to 63 points of damage depending on the number of shots from the shell that actually hit. A headshot can in theory do from 27 to 189 points of damage.

Double Barrel

Body Damage: 9 - 108 (12 shots x 9 damage)
Head Damage: 27 - 324 (12 shots x 27 damage)
Self Harming: N/A

Secondary fire uses two shells to release 12 shots in a wider formation than single barrel, each worth 9 points damage. This may therefore cause 9 to 108 points of damage depending on the number of shots from the shells that actually hit. A headshot can in theory do from 27 to 324 points of damage.

PREDICTION WEAPONS

Prediction weapons have a delay from the time fired to hitting the target, therefore requiring you to predict the location of your opponent when making the shot.

Prop Physics/Gravity Gun

Body Damage: Varies *
Head Damage: Same as body *
Self Harming: Generally no, but objects may be caught and flung back at you. It is possible to suicide on a prop you have flung if you somehow end up back in it's path, such as throwing it up in the air and having it land on you.

* Damage done by the gravity gun varies depending on the following attributes of the object being flung:

Mass

This is not always related to size, some objects are small and heavy, such as a radiator.

Velocity

Heavier objects will lose velocity faster, making longer distance kills harder.

Z-Axis Spin

For example, saw blades spin vertically as they fly.

Angle of Hit on Body

A direct blow hurts more than a glancing one.

Explosive Properties of the Object

Explosive objects yield extra damage, such as explosive barrels, melons etc.

Energy Lost when Hitting a Player

Most props slow down and change angle when they hit a player. If you are the second person hit you will take less. Exceptions include the zombie gibbler (big propellor thing), among other things, that will pass through multiple players without losing energy.

Crossbow

Body Damage: 100
Head Damage: 100
Self Harming: N/A

Orb

Body Damage: 1000
Head Damage: 1000
Self Harming: No, but your orb can be caught in the gravity gun and fired back at you.
Additional feature: Orb tracks opponent upon bounce.

EXPLOSIVES

Explosives yield splash damage that gets gradually less depending on splash radius and the number of units distance away you are upon detonation. Explosives will yeild 5 to 10 points damage at the edge of their splash radius. To see your position and how far the splash radius is, enter the following in your console: cl_showpos 1

Hand Grenade

Maximum Damage: 250
Damage held in Gravity Gun: 225
Splash Radius: 256 units
Self Harming: Yes, if you are within the splash radius you also take damage. Note that grenades may be caught in the gravity gun and thrown back.

SMG/Impact Grenade

Maximum Damage: 200 *
Damage held in Gravity Gun: N/A
Splash Radius: 192 units
Self Harming: Yes, if you are within the splash radius you also take damage.

* SMG Nades appear to do more damage to armor than to health. Until an accurate formula can be determined, the rule of thumb is that they will do approximately 95 damage with a 2x multiplier to any armor affected. SMG Nades also have a small amount of mass and may or may not deliver an additional point of physics damage on a direct hit.

RPG

Maximum Damage: 250
Damage held in Gravity Gun: N/A
Splash Radius: 384 units
Self Harming: Yes, if you are within the splash radius you also take damage.

SLAM (Satchel and Trip Mine Functions)

Maximum Damage: 300
Damage held in Gravity Gun: 200
Splash Radius: 192 units
Self Harming: Yes, if you are within the splash radius you also take damage. Note that satchel slams may be caught in the gravity gun and thrown back.

Explosive Prop (Barrel Example)

A barrel is used as an example here because it is so common and is a good example of an explosive prop.

Maximum Damage: 180 splash damage (plus additional physics damage on a direct hit, typically instant kill).
Damage held in Gravity Gun: 160
Splash Radius: 256 units
Self Harming: Yes, if you are within the splash radius you also take damage. Note that explosive props may be caught in the gravity gun and thrown back.

Source: Steam forum, Walking Target.