I haven't played a call of duty game since black ops in the beginning of 2011, but MW3 is now on its way to my house, any tips for me?

Akimbo FMG's are way over-powered for secondary weapons; use that to your advantage.

Also.. This is only my personal opinion but I play Barebones (game type without the killstreaks) because you don't get the little kids living off their care packages. It really annoys me when they just keep getting attack helicopters and shit just because they got 4 kills.

Best all around gun is the ACR
I user the MP7 with range and rapid fire but it's at a later level.

Shotguns are really good in the game.

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Striker is a boss in this game, kill confirmed gets you alot of XP and using the stinger can get you mass points shooting down people killstreaks

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Yeah, once it arrives, go to Gamestop (Or your closest game store), sell it, and buy Battlefield 3.
MW3 is an Arcade game compared Battlefield 3,and I say that because anyone can be good in MW3. (Given the right gun)
Not in Battlefield, you need a good squad, and need to use strategy to get anywhere in that game.
Most realistic game out there.

If you're the type of person who enjoys running and gunning, you should use this class setup:

WEAPONS:

• MP7 Primary, 2 attachments w/ Silencer + Extended Mags
• FMG9 Secondary, akimbo

GRENADES:

• Any lethal you choose, I prefer C4 or Semtex personally. C4 is for giggles, plus it's useful as fuck.. I find the nades too inconsistent in MW3.
• Tactical is up to you, in the earlier levels I would use concussions because they impair movement. Flashes are also a good choice as they have a huge radius and blind enemies for a long time, however they do not impair movement. Once you unlock it, you should use a portable radar. VERY useful.

PERKS: (Specialist)

• Sleight of Hand Pro:
  This is a given for me, I am constantly rushing and need to reload fast often times. The Pro allows you to swap between your primary and secondary faster, which is helpful in tight situations where a reload isn't possible.
  
• Hardline Pro:
  I use this to unlock my perks faster. The Pro version allows you to get a kill towards your killstreak for every 2 assists you receive. Very useful as well.
  
• Steady Aim Pro:
  This one is kind of up to you. I alternate between this and Sitrep Pro. Sitrep alone is not all that useful, but the Pro allows you to hear enemy footsteps much louder. Steady Aim tightens your spread on your gun, which is useful if you get into a CQB situation where it makes more sense to hipfire rather than aim down sight. The Pro allows you to ready your gun faster after sprinting, which is very useful if you're sprinting all over the map like I tend to be.

KILLSTREAKS: (Specialist)

• 1 Kill: Assassin Pro :
  This is a must, it helps keep you off the radar during UAVs, Advanced UAVs, portable radars, heartbeat sensors, thermal scopes, etc. This in conjunction with a silencer ensures that you can move around the map at your leisure without anybody knowing you're there. The Pro is helpful, but not necessary. It makes it so you are unaffected by EMPs and Counter-UAVs.
  
• 3 Kills: Sitrep Pro:
  This is if you are using Steady Aim Pro as your 3rd perk. If you choose to use Sitrep Pro as your 3rd, a good 3 kill unlock would be Dead Silence as it compliments Sitrep Pro very well. The Pro of Dead Silence is not necessary, but again it proves useful at preventing falling damage. Another useful unlock is Marksman, as it is essentially a legal wallhack. You can see enemies from extreme distances using Marksman, sometimes even through walls/objects. It's a good choice, but it works better on a long range weapon such as an Assault Rifle. If you do decide to use it, ADS a lot - you'll start noticing names popping up in places where you would have never saw the person.
  
• 5 Kills: Scavenger:
  The Pro is not necessary unless you are using it as a default perk. I run Scavenger at 5 kills on all my classes, it's pretty much the only perk other than Sleight of Hand in the first perk category that is worth half a shit. Recon sucks, though Recon Pro can be VERY useful to use during your specialist bonus as even just hitting a single bullet on someone will give you like 15 seconds of real-time monitoring of that person's position.

If you honestly believe that Battlefield 3 has even the slightest resemblance of realism you are highly deluded as to what reality is truly like. The guns, the mechanics, the health system... none of it is realistic. About the most realistic thing about either games are the graphics.

Definitely play Kill Confirmed if you wish to level up quickly, but for fun play Search and Destroy. Use the PP90M1 with silencer and kick when unlocked (quite early around level 25) and the ACR 6.8 when unlocked, very good all-round gun.

Your eventually adapt yourself to your own suited gun, which will most likely be ACR 6.8 or MP7, I advise either FMG9 Akimbo as a secondary or a stinger.

Do challenges to gain xp.

Complete or play campaign mode first so your back and useful to gun games.

Highly deluded, are you serious? You honestly believe MW3 is more realistic then BF3? You obviously haven't played enough, of anything in general. Why is there no such thing as bullet curve/lagg in MW3, but in BF3. You think MW3 is like real war? In BF3 it's a real war, there's ground war, air combat, and vehicular combat. How many of those are in MW3?
C'mon bro get your shit together.

If your going to be sniping use the bolt action snipers rather than the semi auto ones, I seem to get less hitmarkers when doing that

Cmon bro, learn to read. Rofl. I never, repeat: NEVER said MW3 was realistic. I even said "the most realistic thing about either game are the graphics". BF3 is not realistic, have you ever even fired a real gun? They handle NOTHING like the game versions. Bullet drop.. cool, why doesn't wind effect bullets? You think in real life, in real war.. you're going to just get away with going full auto on someone from 200 meters? No. You think you're going to just jump out of a second story window and run off like nothing fucking happened? No. You think you're going to eat 7 or 8 bullets from a high powered Assault Rifle in real life? No. You think you're going to take a couple shots, quickly go prone, and within seconds be fine and ready to go again? No. You think real war has bullet magnetism? No. You think driving a humvee, jet, or tank is as simple as moving a couple joysticks or a mouse around? Hell no. You're one of those brainwashed kids who has come to hate CoD for what it is and seek out every chance to bash it you are given, including bashing other people even when it should just be a playful conversation. I guarantee your reply will be something along the lines of "yeah, well, MW3 has/doesn't have *insert feature/weapon/whatever here*" because you have so very obviously missed the entire point of my post. There is no such thing as a realistic game. You want realism, join the fucking army. You remind me of my girlfriend's brother, he pretends to know everything there is to know about war, guns, and anything remotely in between because he's played a couple war games.

Guys come on, don't start flame wars over video games please. MW3 and BF3 both have their goods and bads, neither is the best out there. Besides the OP didn't even mention BF3 in his post. He was asking for advice about MW3 online.

On Topic: I recommend trying out all the guns and experiment with different perks and such until you build a class that suits you and you work best with. Sometimes I just pick up random guns in the game and experiment with them, makes it a lot more fun and challenging using a random gun.

I never intended to, honest mistake. People who just have to butt in with an unrelated opinion just grind my gears. My bad.