I have a great case for shotguns now that involves a mass opinion of them.  (I am in the united states so other countries might have different google results)

Go onto google and type in "mw3 shotguns" with a space at the end.  That will show you the most googled topic that starts with the phrase "mw3 shotguns".  The first result to show up is mw3 shotguns suck.

Next google "mw3 shotguns are" with a space at the end again.  This one will surprise you.  The first result is mw3 shotguns are over powered.  But when you click on that search you get videos of fails and rants about mw3 shotguns being underpowered.  The first result is entirely sarcastic.  The next is mw3 shotguns are under powered, and the next is mw3 shotguns are useless.

It is the popular opinion of the people that use shotguns in mw3 that they are underpowered.  And the fact that they significantly under perform compared to the other close range primaries in mw3 even when fully leveled, coupled with the fact that most mid and long range weapons also out perform them as well, this also coupled with the fact that shotguns are absolutely useless until they get at least range (the semi and auto shotties with range are okay, except the aa12, there is no redemption for that gun.) Coupled again with the fact that the other primaries in mw3 have stopping power built into their close range damage, makes shotguns an extremely ineffective class by comparison. 

Lets analyze this:

Assault rifles: mw3 m4 is a 3 to 5 bullet kill at 754 rpm and has great accuracy.  Mw2 m4 was a 3-4 bullet kill with stopping power at 776 rpm.  A small decrease in rof from mw2 but identicle close range damage to mw2 with stopping power.  Scar-l and g36 are 3-5 and 3-4 bullet kills respectively and both have fantastic accuracy with kick on.  Then of course there is the type 95's close range damage of 55 per bullet at 1250 rpm burst with rapid fire.  Also there is the 4 bullet kill at range of the Acr which is easy to take advantage of because the gun has so little recoil.  So assault rifles have it pretty good except for the cm901, ak47, and m16.

SMGs:  All kill in 3 bullets up close, and all of them are capable of near 900 rpm fire rates with rapid fire.  The pp90 in particular has a fire rate of 1200 rpm with rapid fire and is capable with range of 3 bullet kills out to 814 inches.  That gives it a .1 second time to kill out to 814 inches.  Then of course there is the mp7, no recoil, high default fire rate, still a 3 bullet kill ( I love this gun).  Smgs have it pretty good also.

Snipers:  The barrett, as50, L118, and MSR all have stopping power strait up built in.  I think it is funny these weapons are designed to be one shot kills so they made it so they have stopping power damage built right in.  These snipers don't need to be leveled to 28 to get a one shot kill to the stomach.  I remember sniping in mw2 with the wa2000 and cold blooded.  It was easy to get one shot kills.  With snipers (unless you are trying to quick scope) you can distance yourself from the fight and be relatively safe while you line up your shots making it easy to hit the chest.  Snipers (besides the dargunov) have it really good in mw3.

LMGs:  After the minimum damage buff they got they are fantastic weapons.  Before the buff only the mk46 and L86 were any good.  Now they are all pretty good (L86 with grip and thermal is awesome).  Lmgs in mw3 are similar to the mw2 high fire rate lmgs like the m240 and mg4 with stopping power.  The mk46 and L86 in particular are capable of good accuracy and a high fire rate 3 bullet kill at any distance. 

Now onto shotguns:  The striker is the same from mw2.  In mw2 it was crap compared to the others so it makes sense that it wouldn't really need to change.  The usas is okay medium fire rate and high close range damage coupled with extra sprint make the gun usable. 

     Now onto the crap.  The aa12 does 15-5 damage stock firing 8 pellets at a rate of 400 rpm.  It has 50 inches less range less than 1/3 the damage at range, and all of its mw2 counter parts ammo problems.  The aa12 is probably the worst gun in mw3 (m16 is slightly better and the dragunov ties it)  Even fully leveled it still terrible. 

     The ksg has 20% less total range, And the mw2 spas with stopping power was a 3 pellet kill 80 inches past where the stock ksg can even get hitmarkers.  It has low range and is terrible untill leveled until at least 23 if not 28.

     The spas with damage has less range, less damage at range, less range to the damage drop and has to be leveled to 28 to even get here.

     The 1887 with damage has less total range, much less range to the damage drop, a much larger spread, less ammo, and no attachments.  The only redeeming quality of the model is its 4 pellet kill at range with damage on. This is offset by its larger spread and the other negatives listed above. 

As you can see the other primaries in mw3 have some redeeming aspect from mw2 in them that makes them a good primary.  But even the fully leveled spas, 1887, ksg, and aa12 are far worse than their mw2 non stopping power counter parts.  Shotguns were unfairly discriminated against in the weapon balance of mw3.

Why put up with this...

When you can do this...