The MW 3 event introduces a new free unlockable Mastery operator skin called Kill Shot. This guide will provide a breakdown of the event, including the different modes you can grind in, the fastest method to unlock the skin, and a review of all the rewards.

Event Categories:  
1. Zombie Bones: These are consumables for the zombie mode and are not relevant if you don't play Zombies.  
2. Armored Zombie Skulls: Collect 600 of these to unlock the Tiny Scream Charms.  
3. Hellhound Skulls: Collect 300 of these to unlock the Dogs of Hell calling card.  
4. Mimic Skulls: Collect 200 of these to unlock the Cursed Ammo, a skin for the ammo box field upgrade.

Modes for Grinding:  
1. Multiplayer: Play Horde Point mode, which spawns different zombies and bosses. However, getting the mimic skulls in this mode can be time-consuming, so it's not recommended.  
2. Zombies: While most skulls are easy to obtain in this mode, finding mimics can be challenging. Other players will also be competing for mimics, making it harder to collect the required amount.  
3. Warzone: This is the most efficient mode for grinding the event. Plunder mode is particularly fast as opening various loot containers, such as weapon cases, medicine cabinets, and cash, will drop piles of bones. Mimic skulls are also obtainable in this mode.

Fastest Method: Plunder Mode in Warzone  
1. Drop into the Seaport District on the map, where there are multiple hotels.  
2. Loot the bottom floor of each hotel, opening weapon cases, medicine cabinets, mirrors, duffel bags, and other containers.  
3. Each loot container will drop a pile of bones, including mimic skulls. On average, you can collect around 80 mimic skulls in about 8 minutes.  
4. If desired, you can exit the game and load back in to continue looting the same area. However, as more players discover this method, competition for loot may increase.

Note: The bones and skulls collected are shared among the entire team. So, if you pick up a stack of mimic skulls, your teammates will also receive seven mimic skulls each.

Review of Rewards:  
1. Armored Zombie Skulls (600): Unlock the Tiny Scream Charms. These are cosmetic charms that can be attached to weapons.  
2. Hellhound Skulls (300): Unlock the Dogs of Hell calling card. This calling card has a zombie-themed design.  
3. Mimic Skulls (200): Unlock the Cursed Ammo, a skin for the ammo box field upgrade. The ammo box now has a flesh-like appearance with glowing eyes.

Master Reward:  
Completing all three challenges (excluding Zombie Bones) will unlock the Kill Shot Mastery operator skin for Swagger. This skin features a red Zombies theme, with a cracked glass mask that reveals one of his eyes.

By focusing on plunder mode in Warzone and looting extensively, players can quickly collect the required number of bones and skulls. The rewards include cosmetic items like charms, calling cards, and a unique ammo box skin. Considering the minimal time investment required, participating in this event is worthwhile for players looking to enhance their in-game aesthetics.