There are many ways to upgrade weapons in the MW3 Zombie. U4GM sifts through a large amount of information and combines some effective methods, such as in-game tasks, certain special items, and maximizing kills. A guide to quickly obtain a large amount of Weapon XP has been compiled for players' reference.

1. Complete Dr. Jensen's retrieval mission.

Complete all the Act One missions and get to the end, where you have to do the Dr. Jensen retrieval mission. This will spawn a special extraction that will bring you to the Dr. Jensen mission. When you go there, it counts as a separate game. It is important to note that completing the Dr. Jensen mission is not necessary for leveling up weapons efficiently, but it can be used as a method to reset the weapon level cap and gain XP faster.

The weapon xp is based on the overall weapon xp gained, so any and all xp counts towards it. Even if you get xp with a weapon that is maxed level and then switch to a new weapon in that same match, you won't be able to get as many levels because, technically, some xp went towards the weapon that was already maxed. Therefore, we need to reset the upper limit of the weapon level when upgrading the weapon.

COD MW3 Zombies Act 1 Mission Tips and Walkthrough

2. Obtain Deadbolt Turret Circuits.

Deadbolt Turret Circuits can help maximize kills and gain XP faster. These can be found by completing infestation nests or infested strongholds. The more circuits you have, the better.

You can look for Deadbolt Turrets throughout the map. Although they spawn in random locations, you only need to look out for the turret icon.

Deadbolt Turret Circuits Locations
- On the blue vent when you get off the plane.
- Whenever you hop down on the truck bed.
- On the blue barrel across from Dr. Jansen.
- In her room after you open the door
- On the box in the little hut past her office.
- Go up the stairs, take a left, on the desk to your left.
- On the wooden crate under the helipad.
- On the base of the radio tower thing crossing the metal bridge from the helipad.

There are two hidden Caches in the water that often spawn some as well. You can also find them as a random item on the ground/tables but that is more rare.

You can also find them on the map in Infested Strongholds and Aether Nests. 
- Look for infestation nests or infested strongholds. These are locations where you can find Deadbolt Turret Circuits.
- Complete infestation nests or infested strongholds to obtain Deadbolt Turret Circuits. The more circuits you have, the better.
- By defeating all enemies inside an Infested Stronghold, players can quickly access several reward caches that can give at least one ammo mod Circuit Board.

For more ammo mods to unlock, please visit MW3 Zombie Boosting.

Use Deadbolt Turret Circuits to power up your turret and maximize your kills. The turret can take out zombies you are not focused on, allowing you to gain XP faster.

3. Use double weapon XP to maximize your XP gains.

 - Obtain double weapon XP tokens. These can be obtained through various means, such as completing challenges or purchasing them with in-game currency.
 - Activate the double weapon XP token by going to the "Create a Class" menu and selecting the weapon you want to level up.
 - Once you have selected the weapon, you will see an option to activate the double weapon XP token. Select this option to activate the token.
 - Play the game mode of your choice and start earning weapon XP. With the double weapon XP token activated, you will earn twice as much weapon XP as you normally would.

It is important to note that double weapon XP tokens have a limited duration, so make sure to use them strategically to maximize your weapon leveling progress.

A double xp event is going on from the 22nd to the 27th for the player, weapon, and BP. Great time to max weapons, then.

4. Complete contracts to get up to 750 kills and max out all your weapon XP for the game.

It takes 2,750 XP per weapon level. Each base zombie kill gets you 20 XP (without Double XP). This means you get a weapon level every ~138 zombie kills. After you reach that 750 zombie kill limit, you should have obtained 15,000 XP, which should give you 5.45 weapon levels.

Pretty sure that after you get the 750 or so kills it takes to reach the cap, it doesn't matter how you get kills. They will all give the reduced xp.

A double XP token would double this output, meaning you should be hitting the level 11/12 mark by the time you reach the 750 kill limit, as there are bonus points, special zombies, contracts, loot XP, etc., that vary this.

If you think it is difficult to complete 750 kills, you can buy MW3 Boosting from a professional team.

5. Start an Outlast contract.

Start an Outlast contract to maximize spawns. Start by activating the Outlast contract, open a map and ping the contract closest to your location. Contracts are marked on the Tac map with a light blue walkie-talkie-shaped icon with a picture of a satellite dish, which can be found in various threat levels.

The closer you are to the contract, the more spawns you get. Let the aggression go up a bit, step out of the area, let it go back down, and then come right back to maximize spawns.

You must stay in the designated area and fight against the incoming zombie hordes until the tracker on the left reaches 100% to complete the contract.

Once you have a decent amount of kills, end the game and Pack-a-Punch your weapon.

6. Start the Dr. Jensen mission.

Hop into the special extraction that brings you to the Dr. Jensen mission. This will reset your weapon level cap, allowing you to gain XP faster.

Complete the Dr. Jensen mission and loot around the area if you want to. Rush to the objective to complete the mission quickly.

The "Where's Dr. Jansen" mission Guide

Repeat steps 1-6 to continue leveling up your weapons efficiently.

If you don't want to repeat the above steps, you can also do a spore control mission and turn the tightest group of spores red but not blow them up. Zombies will spawn infinitely. But the efficiency of manual killing is not as high as Deadbolt Turret.

Tips:
If you're doing infestations, use phd perk to not get damage from the gas. Also recommend using a pap'd grenade launcher, which makes clearing infestations easy.

When you were taking damage and couldn’t figure out from where, it was most likely a glitched sniper that’s floating 300ft off the ground. It’s a glitch where a mercenary sniper gets stuck in the sky. You just have to look up and kill him.

If you want to get the deadbolts faster, ensure you have stamina up and death perception and go to tier 3 and find the crystal loot boxes. You can also pair up Jugg and Speed Cola, but you generally need a 3 armor plate and the two perks.

More reading
MW3 Zombie Firearms Upgrade Test - P90
COD MW3 Zombies Act 1 Mission Tips and Walkthrough
COD MW3 Zombies Act 2 Mission Tips
COD MW3 Zombies Act 3 Mission Best Completion Tips