Re: Shotguns.

This one of those area where the theory and the practice are somewhat divergent. For a sort of combination of all the reasons listed, the average US infantry company keeps some shotguns in its arms room. And they tend to be very mechanically easy to use ones, so the training premium is pretty low.

Buuuuttt....

Their infrequent use creates a circular effect where they are harder to use for anything but niche roles.

The system is not set up to pump shotgun shells forward in the manner of 5.56 and 7.62, and "specialty" shells are unlikely to be available in most common circumstances. Every guy with a shotgun is not running around with a slug, buck, less-than-lethal and the like in a shell gourmets choice to pull from his large selection. He likely has a small belt/harness on his kit with a handful of buckshot quick to hand. Some, but not firefight sustaining quantities of the stuff.

And if he should happen to have a specialty use - less-than-lethal let's say - even in normal counter-insurgency he won't have many, they might be in his assault pack or gun truck, or they may be sitting back in the company trains for "when we need them". Which may be bureaucracy, and may just be sensible not wanting to add another pound of ammunition you'll only use once in a blue moon to the 60 pounds of minimum fighting load already being carried. And who knows when more of this rare ammunition will work it's way forward. Maybe the deliberate riot response units in the Balkans will have those rubber rounds in abumdance, but not a random patrol in Kandahar.

Then the gun itself is obnoxious. It's not as if the man carrying it only has a shotgun - his primary weapon is still his rifle. So the gun is either in a truck, or on an extra sling, or if the go-to-war-money has been particularly bountiful in a special shotgun back sheath. (This, incidentally, is the reasoning behind the under-attachment theory). You can imagine that carrying the damn thing around is extra weight and one more awkward bundle - and almost never in a position where the shooter isn't just going to use his rifle unless it's deliberate. Between "in my hands and one thumb switch away from shooting" versus "let go of rifle, draw shotgun, probably load shotgun (loaded shotguns bouncing and jangling in random directions off a moving soldier, the ground, etc are maybe not a great idea for him or his mates), then use shotgun" you can imagine which one wins.

So then we get to breaching. The shotgun is in a really fine niche here. You are basically blasting a pattern so that the lock no longer is held tightly by the door. Which implies that the door itself is of lighter construction. A great many doors can be breached quicker and easier by non-shotgun methods, such as "testing if they are open", "breaking a window and reaching through", "just lift it off it's hinges" and of course "get the big guy to do it." Those that can't in many cases are not susceptible to a shotgun breach to begin with - and sometimes metal lockplates send a bunch of bits of buck flying back out and around the courtyard in ways that make everyone cringe. The line between "kick" and "we need to hit this with real breaching tools or demo (or the ever popular find/make a different entrance)" doesn't hold a lot of shotgun space, no matter how cool it seems.

And then we get to CQB. The theory of this is "shotgun, close range, brilliant." But....first, there's that whole part about still having a rifle. And that most people are trained (at least summarily) in shooting said rifle for CQB, while they almost certainly aren't beyond point-pull for the shotgun. And since good CQB involves your own side flowing through the building/trench/whatever with a decent tempo rather than getting bogged down in a firefight (you'll lose), there's a decent chance that there are friendlies in the area you're aiming. If you are aiming three feet off their nose, they are going to be way happier if it is with a rifle you trained with rather than a shotgun you haven't. And since in a military context anyone standing up in said room/trench/etc. is probably getting shot four to eight times as each man tracks his weapon through his sector and fires twice on the way...the stopping power of an individual small cartridge becomes less of an issue. Plus, ammo. When attacking a trench, lots of bullets get sent down the trench just to make sure no one decides to pop around the corner or even consider using that bend - then the frag goes over, and the you move down the next bend. Repeat. You almost certainly aren't carrying enough shotgun ammo to do that, nor is the magazine size large enough for a sustained advance.

So - shotgun theory, hot. Shotgun execution - less practical than video games and individual theory would think.