I has question?

No.

The weapon damage is based on the amount of time you are in game. You can spray all you want with it, but it won't increase the damage to the gun.

I'm 90% sure about that.

Well then, I find the gun turning red as you spray very misleading then.

Damage is based on the time the bag is in use. For example, I equip an m4 and a de. When I play if I only use the m4 then the de won't receive any damage. And if I drop the m4 immediately and pick up another rifle it'll still deal damage to the m4 I originally had in the bag.

Other people picking up your gun does not damage it, if you drop the gun and only use your pistol the rifle will receive little to no damage even if someone else picks it up, because your "main" weapon slot in the bag wasn't used it doesn't deal damage to it.

In short damage is based on your time of use, not on spray, and not by anyone else.