If(Range>Effective_Range)
then
Base_Accuracy - (Range-Effective_Range)^2/Muzzle_Velocity

else
Base_Accuracy - (Range-Effective_Range)^2/Mobility
end

Effective Range is where the weapon is most effective.
Muzzle velocity defines how quickly the accuracy will decrease when shooting further than the effective range.
Base Accuracy is the accuracy of the weapon.
Mobility is how mobile your weapon is. Bigger guns usually have lower mobility that gives a penalty the closer the target is.

Chance to hit =
(Marksmanship+Merc_Level)*(Weapon_Accuracy/100-Penalties+Bonuses)

Marksmanship and Merc_Level speaks for itself.
Weapon_Accuracy has its own formula dependent on weapon type and attachments.
Penalties can be i.e. injuries, morale, automatic fire or damaged gear etc.
Bonuses can be i.e. stance, scope or gear etc.

If(Range>Effective_Range)
then
Base_Damage - (Range-Effective_Range)^2/Damage_Falloff

else
Base_Damage
end

Base_Damage is the damage value of the weapon
Damage_Falloff is the Damage Falloff value of the weapon, of course, but beware, higher the better (damage falls slower with distance)