The military uses them for breaching... their mainstay is still the m4 carbine, even after the breach. If combat was nothing more than clearing houses and building, I'm sure that shot guns would be more viable, but they are too limited. I used to schedule ranges for a mobilization platform and I can tell you, the shotgun is not used for standard combat, nor are soldiers qualifying for that firearm on ranges. Of the hundreds of ranges I scheduled, only once was the ammo requested for 12 guage buckshot. Notice how light they make shotguns in FPSs? Pick a loaded combat shotgun up sometime and tell me you'd rather have that over a carbine.