To give this topic another boost:
Scarhands last suggestion:
PHP Code:
((Damage_Given / Time_Played) * (Hits / Shots)) + ((Damage_Given / Time_Played) * (Headshots / Hits)) - (Damage_Received / Hits)
Remark made by Paul and myself : don't use the time inside of the formula because afterwards we want to multiply the achieved rate with (3600 / Time_Played) -> 3600 being the maximum time for which rate is given.
It took a while before I could understand your problem with this method Scarhand 
The only difference between our 2 methods is the size of the result.
There is in no way a change within the accuracy or headshot ratio will occur -> this is because of the usage of a multiplication with the (Damage_Given / Time_Played) factor.
So when the (3600/Time_Played) factor is inserted in your formula but without your Time_Played we get the following:
PHP Code:
(Damage_Given * (3600 / Time_Played) * (Hits / Shots)) + (Damage_Given * (3600 / Time_Played) * (Headshots / Hits)) - ((Damage_Received / Hits) * (3600 / Time_Played))
(also saw that you didn't add Time_Played at the Damage_Received side but I suppose that was a mistake?)
So this basically comes down to something similar but with a change in magnitude.
A proposition that was made by Paul :
Instead of punishing the use of heavy weapons, artillery or airstrikes we should increase the reward for using the proclaimed "skillful" weapons.
But the current formula doesn't really provide a way of doing this besides the (Damage_Received / Hits) factor.
As such we also need to create a list of weapons we want to include in the accuracy and headshot ratio calculation.
WANTED: new opinions (on topic), who knows perhaps providing the ultimate rating formula could give you some advantages