The common weapon used on board ship is a shotgun. I see lots of marine grade shotguns - and few marine grade carbines.
None of the commercial "marine grade" shotguns are in military use. Their military counterparts are black/grey phosphate finished just like military rifles.
The use of shotguns on board ships parallels the application at home. Tight quarters, close ranges, and the ability to select the load gives shotguns an advantage. The streamlined profile of shotguns, like the Ithaca, Mossberg, or 870, is certainly a lot easier to use, and will hang up less on obstructions than a iron sighted, pistol gripped, magazine fed carbine.
Infantry men have no trouble climbing in and out of the even tighter confines of trucks, APCs, and other land vehicles with carbines.
Any weapon should be optimized to the average shooter in HD.
Interestingly, many shotgun manufacturers are moving to AR-15 style collapsible butt stocks so that the pump shotgun's LOP may be easily adjusted to shooters of various sizes. .223 / 5.56 Carbines are lighter, have lower recoil, less muzzle climb, and are faster to make follow-up shots with. The average shooter can shoot a .223 carbine much easier than a 12 ga shotgun.
Penetration of flimsy sheetrock construction is also a consideration.
Buckshot is going to penetrate sheet rock just as much .223 hunting loads.
Varmint loads are notorious for wild richochets.
People used to assume that .223 carbines would penetrate more than pistol caliber carbines, but scientific testing showed that wasn't true. I've seen #4 buckshot ricochet as it shattered plate glass that it was skip fired off of. The ricochet argument can go both ways.
Since family members may be just an inch of sheetrock away, it has a lot to do with caliber selection.
Even bird shot will go through sheet rock. Once again, ANY load capable of incapacitating a 200+ lb mammal is going to penetrate multiple layers of sheet rock.
What you choose does reflect how you intend to use it in your circumstances - but when you do need stopping power, the 12 ga slug is an option. A 5.56 doesn't guarantee it as well.
First you argue for a 12 ga with shot for lower sheetrock penetration than a 5.56 (which usually isn't the case anyway), and now you're arguing for 12 ga slugs for better stopping power. Again, First you advocate the shotgun as being easier to use, and now you're advocating some of the heaviest recoiling loads commonly fired from any long arm in North America. Which is it?
I believe the handling, use, versatility, and purchase of a HD shotgun offers a decided advantage over a military battle rifle that is hard to maintain, buy ammo for, or train with.
Really? I can field strip my AR-15s with no tools. It takes punches to field strip an 870 or a 500/590. There are plenty of online and print guides that show and explain how to maintain an AR-15. The US Military trains thousands of troops who've never even touched a gun in their life on how to use and and maintain an M-16/M-4. The US Air Force does it in just a few hours. AR-15s are not hard to maintain.
How is it hard to buy ammo for an AR-15? Seriously, I'd like to know. I can buy 08 Lake City headstamp 5.56 for $9 / box of 20 at the local gun store. Even cheap 12 Ga bird shot is $7 or $8 for a box of 25. Buckshot is $6 or more per box of 5.
How is it hard to train with an AR-15? Most indoor ranges have at least one rifle bay, and even though it may only be 25 yards it's still a place to shoot a rifle. They'll usually let you shoot shotguns too, but what's the advantage to shooting a shooting from fixed firing position shooting lanes?
Most civilian homeowners aren't prepared for the cost of purchase, expense, and use - but many of them can buy a shotgun, go to the range, and shoot a case of shells, which can be good familiarization. It far more likely to happen than signing up for a weekend at a CQC battle course.
Ohh, so you need a carbine course to learn to shoot an AR-15, but just going to the range on your own is sufficient training with a longer, heavier, and harder recoiling shotgun? So no one needs to take a defensive shotgun course to have training equivalent to what they'd receive in a defensive carbine course?
Having used AR's for 22 years, and owned a HK91, the last weapon I would recommend for home use is either.
I wouldn't recommend an HK-91 either. It's another heavy gun, with heavy recoil, and slower follow up shots.
Like 4 wheeling an Escalade off road on a camping trip, you may not like the cost, especially if that's all you ever do with it.
Escalades weren't built for off road use. You can buy a base model Toyota Landcruiser for the same money as an Escalade and have an incredible off road vehicle. In the same way, AR-15s are actually built to be run hard and employed for defensive use. A Browning BAR hunting rifle used for nothing but HD would be equivalent to using an Escalade only as an off road vehicle.
Home defense shotguns are the better choice, and the average family will get more from them.
To use your own automotive metaphor, you can play off road with a stock base model Jeep Wrangler . Alternately you can buy a Wrangler Rubicon off the lot, or spend quite a bit of money upgrading the base Wrangler and have a purpose built off road vehicle. Likewise you can buy an 870 Express and probably be just fine. Alternately you can buy an 870P or send your Express off to someone like Scattergun Tech, and get a purpose built fighting shotgun with beefed up internals.