I must remind you that all my testing was done on PS3 and its possible that there is a small diference in rate of fire on Xbox considering that most dont match the game files.
Having said that, an AP pistol kills a level 324 player in 9 body shots, time between shots is 0.100ms (just like the game code) so its a 0.800 ttk. The special carbine its 8 body shots at around 0.116ms so its 0.812 ttk. Advanced is 7 shots at around 0.126ms so its 0.756 ttk, theres barely any diference between AR´s and AP pistol. However, the AP starts losing damage over range before the AR's and its less accurate, so AR's and MG's have the advantage.
What makes AP unbalanced for me its as a drive-by weapon, it overpowers all other pistols and micro smg (even the other smg's as a run & gun weapon).
For balancing Rockstar should buff all other pistols and smg's as right now they are useless.
In GTA smg's were always underpowered AR's, the only usefull one was the micro smg for drive-by shooting. It looks to me that Rockstar designed the game so that we get better weapons as we level so there is no need for balance, well... until they started adding DLC weapons without level requirement to unlock. Personaly i would like a bit more balancing, whats the point on adding a Heavy shotgun that gets beat by the Combat shotgun? Im not even going to talk about the useless pistols...
Its not the weapons that are weak, its the absurd amount of health players have (around 230hp), add the fact that if you hit the limbs its -20% damage, -75% for the Heavy sniper (yes a heavy sniper takes 4 shots to kill if you hit the limbs). For this reason pro players go for headshots only but i find it very annoying that this game turns into a headshot only mode.