So over the last few weeks, there's been a bit of confusion and indecisiveness over whether to upgrade to Mk.2 weapons, and not only that, but whether it's worth reducing your overall ammunition and magazine capacity, in favour of special ammo which may (or may not) be more effective against certain things. I'll say now ahead of time - every Mk.2 weapon performs better than their original counterpart in at least one field.
These tests took about 2 weeks to do - which was probably five days in GTA V and Online testing things and then the rest of it was making my notes and drawing up a table for easier reading.
Anyway, enough with the spiel, onto the important stuff... this post won't be short, hence all the spoilers. If that's too much for your eyes, I'd suggest using the table (spreadsheet) version linked below.
GTA V and Online - Special Ammo Types - Testing & Results
Some important notes - please check first:
Stock ammunition, known as "base damage":
! Same damage to vehicles and personnel, as with other firearms in the game.
Tracer rounds, "Bullets with bright visible markers that match the tint of the gun - random chance":
! Only differences from stock ammunition are the particles / effects of the tracers and max ammo count is different, as follows:
Overall / Magazine (Total):
Pistol Mk.2 :- 348 / 12 (360)
SMG Mk.2 :- 330 / 30 (360)
Assault Rifle Mk.2 :- 330 / 30 (360)
Carbine Rifle Mk.2 :- 330 / 30 (360)
Combat MG Mk.2 :- 500 / 100 (600)
Hollow Point rounds, "Increased damage to targets without body armour":
! If a player or ped is wearing body armour, these bullets will do less than base damage - they only get a boost in damage when they're depleting health.
Incendiary rounds, "A random chance to set targets on fire when shot":
! Fire will remove an extra 165-175 health from peds (NPCs), but its much less from players.
! STK (Shots To Kill) has been calculated with enemies that were fireproof (in case none of your bullets set the target on fire).
Armour Piercing (AP) rounds, "Increased penetration of body armor":
! AP bullets will do 10% extra damage of the Damage Per Round to ped & player armour.
! Damage is dealt to both health and armour at the same time.
! STK is identical whether they're wearing armour or not usually, as the player's health will be fully depleted before their armour is.
! No extra effectiveness against vehicles, armoured or not.
Full Metal Jacket (FMJ) rounds, "Increased damage to vehicles. Also penetrates bullet resistant glass":
! Will break through armoured vehicle's glass and do damage to the person inside in just one shot.
! FMJ rounds do exactly double (2x) base damage to vehicles.
Explosive rounds, "KaBOOM":
Hopefully this information will have been of some use to people. Good luck in your rampages!
Overall, here's my recommendations for weapons and ammo types :-
Pistol Mk.2 = Hollow Point rounds
SMG Mk.2 = FMJ rounds
Assault Rifle Mk.2 = Armour Piercing rounds
Carbine Rifle Mk.2 = Standard ammunition
Combat MG Mk.2 = Incendiary rounds or standard ammunition
Heavy Sniper Mk.2 = Explosive rounds.