Best Class in MW3?

Primary: PP90M1 with Kick and Extended Mags
Secondary: .44 Mag with Tactical Knife
Lethal: Frag
Tactical: Portable Radar (if you're not high enough level to have this unlocked yet then just use concussion grenades)
Perk 1: Sleight of Hand Pro
Perk 2: Assassin Pro
Perk 3: Steady Aim Pro

Strike Package: Specialist
Specialist Perk 1: Scavenger Pro
Specialist Perk 2: Blind Eye Pro
Specialist Perk 3: Blast Shield Pro

Death Streak: Martyrdom

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Another Class I use:

Primary: MP7 with Range and Rapid Fire
Secondary: SMAW (rocket launcher)
Lethal: Frag
Tactical: Concussion
Perk 1: Blind Eye Pro
Perk 2: Blast Shield Pro
Perk 3: Sit Rep Pro

Strike Package: Support
Support Pointstreak 1: Ballistic Vests
Support Pointstreak 2: Recon Drone
Support Pointstreak 3: Escort Airdrop

Death Streak: Martyrdom

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Give those a go and see how you do. You have to play each weapon to it's strengths though and each class is made for a different playing style. The class only has a part in how well you do online, they can really hinder you if set up wrong but you have to know the game to do well. It'd be worth watching some videos and looking at what successful players are doing that you aren't, where they're looking, what they're doing, decisions they make as well as how they handle themselves in fights. Your proficiency and attachment should be catered to the weapon itself and how you're going to use it. Your perks should be tailored to both your gun and how you're going to play the game. That can go down to game modes, how you move about the map, if you're offensive or defensive or many other things. The strike package is entirely down to your play style but you should set it up so it's going to get the best use from you. If you're going for a Support strike package then go for rewards that you'll be able to get and will be most beneficial to your team. If you're going specialist then your goal is to set things up to help you survive to get yourself up to that 7 or 8 kill streak so you can unlock all the perks. Don't pick the same perks for all classes, look at what the perks do and look at your weapons you want to use. Once you start to learn them you'll be able to think up classes you want to try and after that it's just a case of seeing what works for you and what doesn't.

Also learn the maps, try learning the general places where people spawn and the nature of the spawn system so you can predict things that will happen and control it to some degree, learn where the popular camping spots are, busy areas and good flanking spots. When you get killed, watch the kill cam so you can see if you did anything wrong and try to see how you could have avoided that death. You just need to do lots of little things combined with a few bigger and more obvious things to learn the game so you can understand it and get better killstreaks. If you want more kills then you'll have to do this and remember that the game has a flow to it, you have to get into your own flow to do well. Oh and remember to tweak your sensitivity so it's just right for you and I'd suggest turning colour blind assist on because markers on the map and player names show up easier.

I'm thinking of getting this game, is it worth it or will it die soon? any ideas if there's a new COD in the making? I'm feeling like doing some casual mindless killing, and TF2 doesnt fit that need for me.

Assasin Cross; always the best

Pyro

The Quitter

I imagine you'll be fine as long as you're wearing a sweet hat.