Pump shotguns are already a huge chalenge with their slow fire rate and limited range.  The spas, 1887, ksg, and aa12 account for 4 of your 9 worst performing weapons.  Most of those other 9 weapons you don't have as many kills with so their k/d might be much higher once leveled and used extensively.  So lets just say that you pump shotguns and the aa12 are tied for your worst performing weapons along with the pm9.  Do you think that on these small maps that all of your pump shotguns and the aa12 should be your worst performing weapons. 

Mw3 maps are primarily close and mid range.  It is possible to control all your engagements to keep enemies within 15 meters of you.  So why is it that the guns that should have the advantage in those ranges perform worse than the other primaries in mw3?  Its because they don't have the advantage.  All shotguns save the striker need a buff to even claim mw3 has any sense of blance.

What I don't understand is why people get mad about shotguns and not snipers.  They are both a one shot kill but shotguns only work for a certain distence while snipers can one shot kill at any range.  Whats even more fun is that snipers don't have to wory about map navigation or leveling their weapon to get good games.  Yet no body even cares when they get sniped by a dude across the map.  The quality of aim required to get a 15 meter one shot kill with a pump shotgun is better than almost any practical sniper shot.  Yet shotguns catch the hate.

I play primarily slayer games where kills count for score that is part of the reason.  But more to the point k/d ratio tells how a weapon does in a typical encounter in mw3.  The higher the k/d a weapon has the better it performs in a majority of situations in mw3.  Yes you can get high scores by going 40-40 but if you take a better weapon and go 60-20 it will be better for you and your team.  So if you get into 80 gunfights in a game and the shotgun goes 40-40 and the smg goes 60-20 its clear to see which one helps the team more.

For objective games a higher k/d weapon will statisticly be able to defend or attack objectives generally better than a low k/d weapon, making it easier to win.  This works with weapons that generally support the same play style.  Obviously its hard to compare sniper k/d to smgs k/d because the playstyle is so different.  But shotguns to smgs and ARs is a fairly easy comparison. 

To really understand this I'll put it simply, if you change nothing but your primary weapon (playstyle, map, and tactics stay the same) The other primaries will do better than shotguns in mw3.