1. Mileage = how much fuel / 1 Elerium tanked
2. Skill = what skill is used to calculate accuracy
3. POWER Bonus: A bonus to the POWER value listed next to ammo/weapon. So if it's, say, 30, and the bonus = Strength * 0.5, the effective power value for the weapon, when someone with 40 Str is using it, will be 50.
4. Extra damage - amount of extra damage done, after the normal calcs are finished. 'lethal' means that much of the damage inflicted (stun or smoke) will be inflicted again as HP damage, eg. if you shoot someone and inflict 10 Stun damage, he's going to take 5 HP damage as well, if the value for extra damage was 50%. Extra damage 'stun' inflicted by lethal weapons is different a bit, since they generally only inflict Stun if they inflict Fatal Wounds; this property multiplies that stun damage.
5. Extra pain - afair the weapon does extra Morale damage, ie. multiplies the Morale damage normally inflicted by damaging HP.
6. "Special" is just a category to denote special properties of a weapon.