The title serves a dual purpose:  I encourage people to talk about their favorite weapon(s), but the OP isn't necessarily about mine.  Read on.

I just finished playing CoD4 too late into the night.  Today I was having a terrible connection issue, and the lag was making me miss all my targets with my silenced M16A4.  (I was running my stealth class, and I haven't re-unlocked the P90, so I'm defaulting to the M16 which I love.) 

Since I couldn't just QUIT -- surrender the match? never! -- I switched to the default Demolitions class (W1200 shotgun).  Not that I hadn't ever used the shotgun before, but I hadn't given it a serious try.  I figured it was only good for like 10 feet away or less -- basically if indoors in the same room -- and anything further away could own you with SMGs.  And it's almost knife range anyway, right?  

WRONG.  

The W1200 was much more effective than I thought, getting kills at quite respectable distances (though still close range obviously -- it IS a shotgun).  Downpour was the map I happened to be on, which is in retrospect one of the best maps I could have been on for this epiphany.  Although there are few buildings of any real size, the map doesn't have much in the way of big areas of open space -- it's broken up by small buildings, walls, etc., perfect for a shotgun that's vulnerable in large areas.  (An opposite example might be the map with a downed helicopter in it -- lots of big buildings but lots of large, relatively open-field areas.  The buildings are OK for shotguns but good luck roaming around the map.)

So I was running around in Downpour, still laggy but not ridiculously so, and the lag didn't matter so much because the shotgun (A) made me engage at closer range and (B) was easier to hit the enemy with at close range.  Also the Demolitions class has Extreme Conditioning (you can sprint twice as long) and the shotgun naturally allows high maneuverability; I love running around a lot so I was blown away at how much I could run circles around these guys.  I actually faked someone into thinking I was retreating, ran through a building (the tractor garage), out the other side, circled the building and saw him going inside and I was about to blow his ass off when some dude sniped me

I actually ended the round (it was Free-for-All) with a very respectable score (for me).  

Next thing I did of note was a Team Deathmatch on that map that's basically one huge corridor with buildings running down each side, and side streets at the end making "L" shapes.  It seemed like half the people on both sides were sniping, and I was running down to their base over and over again killing everything I saw until they got me.  I was commonly getting UAVs before going down and I got at least a couple aristrikes, no helicopter though.  I was on a RAMPAGE.  We won, and I had 16k/8d, tied with someone else, I assume our team's best sniper.  [edit:  the all-caps seems kind of excessive for a 16/8 ratio, but I got 3 deaths in a row at the start from being stupid, so it felt like 16/5.]

I was constantly getting one hit kills, WITHOUT Stopping Power, and that's what the really fun part was.  Once I even got TWO people in one shot, that was epic.  Juggernaut?  Shotgun don't care.  (Much.  If it's point blank range, he's going down, but at longer ranges probably not.)  

Now, I'm not saying that the shotgun is now my favorite weapon.  I still love the M16A4.  The P90 still rocks.  And I remember how amazed I was when I first tried out the Uzi with Sleight of Hand (fast-reload perk) -- but although the Uzi is still fun, the awe I felt wore off pretty quickly.  Still, I don't think I'm ever going to dismiss the shotgun class again.  

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Now, for those of you about to run out and try the shotguns:  some advice.  The 1200 takes a short while to chamber the next round.  If you miss the first shot and your opponent doesn't miss you, you will probably die before getting a second try.  (If both of you are running around each other getting confused, it's a crap shoot as always.)  But if you are facing multiple opponents charging out of a building at you (or you're charging in), you can usually kill one and get the other shot off in time to kill his partner.  

Also, look down the sights whenever you can, since this will improve the gun's accuracy and maybe range?  Unless the guy is literally in your face and you need to let go of aiming just to follow him, you should always have your sights up when you think you might need to fire this gun.  

The M1014 shotgun has a faster fire rate, but a tiny clip.  (Only four shells as opposed to the W1200's seven.)  And worse, this smaller clip is magnified by the fact that the shotgun is weaker, meaning it will more often take two shots to kill an enemy.  This can mean that you're taking almost twice as many shots with almost half the clip.  And this is assuming the enemy hasn't killed you while in between the first and second shots.  (I haven't "researched" this, so if you feel I'm badmouthing the 1014 feel free to speak up.)  

So in conclusion, I admit I might be biased, but I recommend the W1200.  Recommended perks:  since the 1200 is capable of one hit kills without Stopping Power, right now I'm recommending Juggernaut to help make sure you live long enough to get that one shot off.  But I've only been using it for a day.  

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Remember:  Forget sniping.  Real men OHK with shotguns.   

I forgot to add something: when I say I won't ever dismiss the shotgun class again, that includes the fact that I used to scoff when I saw people running around with shotties; even if one managed to kill me I'd think "just wait until I get more than 10 feet away bub". No more. The shotgun is a weapon to be respected -- along with its users -- unless you catch the sucker at medium or of course long range.

Ak-47

OMG I do a huge post about my discovery of the shotgun's awesomeness, and mini strategy guide, and I get a five character response.
kidding

Mine is M16 with Red Dot. Accurate and powerful, easily the best weapon IMO.

Talking of shotty's though... M1014 is a great gun for playing hardcore. Just make sure you have sleight of hand, and you should be set for the map.

My shotty class -
M1014 (Gold)
RPG x2
Sleight of Hand
Extreme Conditioning (S&D/Sabotage) / Steady Aim (Anything else)

:D.

Bump to try to get more mileage out of this before I admit it's dead. Is Carl really the only one with substantial commentary on either shotguns or favorite weapons in CoD4?

AA-12 is definitely my favorite MW2 weapon.

I favour the M16 with Red dot sight. Probably one of the most useful weapons in the game (slightly overpowered too).

But the Shotguns are surprisingly good. I didn't bother to use any until about 4 months after I got into multiplayer where I realised the only way I was going to level up faster was by getting the points for all the weapons and I started out using the shotgun only on 2 maps (I forgot their names, but the smallest map and the map with 2 solid areas, outdoor and indoor and the indoor part has the little roofless square where most of action in HQ tends to happen, and it has that dinky little shed in one of the corners). But then I started using the overkill perk and it is a very good weapon if teamed up with a mid-long range weapon, or even on it's own if you know how to sneak around.

M16 with ACOG scope, the distance is amazing. Practically sniping but better.