Range modifier :
+30 - Point blank range (3 meters)
+10 - Short range (half range)
+0 - Standard attack
-10 - Long range (double range)
-30 - Extreme range (triple range)
Circumstance modifier :
+30 - Surprised target
+20 - Stunned target
+20 - Melee outnumbered 3 to 1
+10 - Prone target
+10 - Melee outnumbered 3 to 1
+10 - Higher ground
-10 - Attacking while knocked down
-10 - Fatigued
-10 - Mud/water/heavy rain
-20 - Called location shot
-20 - Shooting into melee
-20 - Dodging while prone
-20 - Close combat in darkness
-20 - Shooting in fog, mist, or shadow
-20 - Firing a weapon without the talent
-30 - Deep snow or similar
-30 - Shooting at completely concealed target
-30 - Shooting in darkness
Note that there are no penalties for firing into partial cover - simply a chance that the shot will strike the cover (which is treated as extra armour for the target, usually protecting the leg and body locations)
Especially large or small targets also have a modifier of +30 to -30.
Modifiers combine to a maximum bonus or penalty of +60 to -60 on any single test.