Shot guns were not abundantly deployed. With modern loads they were used to weak or take door hinges/locks off .
Used in some cases as crowd control with less than lethal loads. Building clearing was and is still done with the Saw249 and M4.
Shot guns and hand guns are not primary weapon in the US infantry.
They have worked on the deployment of shot guns many times and it has always come up short. The Benelli M4 was suppose to be an answer but it failed in the real world.
The newest idea has been around awhile the M26 Modular Shotgun .
http://kitup.military.com/2012/02/army-fields-shotgun-101st-airborne.html
While it plays well in the movies and sounds sexy shot guns are not combat weapons.
When not being used in breaching and crowd control they are mainly deployed in close quarters only and even then not that often.
The breaching loads are very effective When used properly. But they are issued in addition to other weapons not a replacement.
When we turned in the old Winchester shot guns you would be surprised at how good a condition the were in from lack of use.
4 man fire team ( In some cases M9 are issued also)
2-M4 (Some cases one M4 maybe swapped for a M14)
M4 with 203 (203 is 40mm grenade launcher who the heck needs a shotgun)
SAW 249 (may at times be swapped for a 240)
No one is giving any of them up for a shot gun
Keep in mind what a combat fire team does and what you will be doing is a different world. You will not have the equipment they use. No one is swapping a SAW 249 for a 12ga shot gun. Except the air force when they are bored and ride around shooting dogs.