-The large size makes it harder to maneuver through doorways, hallways, around furniture, etc.
If you're having trouble moving through your house with a shotgun up to your shoulder, having a sidearm in ready position is gonna be no less easy. any obstacle causing you to remove your shotgun from ready position is gonna do the same to a pistol held out in front of you. practically speaking, a 18" barreled pump, isn't going to significantly decrease maneuverability.
-Length of the weapon provides more leverage and grabbing surface for an attacker should they get close enough to get a hand on your weapon. If you end up wrestling with an attacker you need to gain distance from them just to be able to point the barrel in their direction.
this is kind of two points. the first seems silly to me, since having two hands on the gun, plus having it butted into your shoulder is going to give you GREATER resistance to somebody else moving it off target. I'd argue that it's at the very least no worse than a sidearm, and in my best estimation, better.
If you end up wrestling with an attacker, it will be true that the shotgun will require you to regain more space between you to use effectively, but I'd suggest that the false sense of security being able to fire a sidearm at off angles gives you is going to lead to a greater risk of over-penetration injuries. Since at that range you're looking at an exit wound with even 9mm rounds. Where does that bullet go when it exits the bad guy? into you? into the room next to you? into somebody else? if you're wrestling with somebody, pulling a trigger probably isn't your best option anyways: calling for help may be number one. Not always an option, but if you're ever in a gun fight, bring a friend.
And I think it might be EASIER to twist a sidearm out of somebody's hand than a shotgun. more points of contact with a shotgun, and greater ability to hold onto it.
-Requires 2 hands to operate. You can't open a door without taking 1 hand off your weapon. If you are tangled up with an attacker, you have 1 shot and then you have to choose between using your offhand to fight off the attacker or operate your weapon. What if your attacker has a knife and they are able to grab the barrel with 1 hand and twist it away from themselves as you fire a shot? Then you are either fighting them with 1 hand vs 1 hand with a knife, or you are trying to operate your weapon with both hands while they are stabbing you in the face. The same goes for if the attacker has a pistol. This is especially a problem if you face a physically stronger attacker.
First, I can fire my shotgun accurately with one hand butting it up to my shoulder. If you're dumb enough to sacrifice your trigger hand to open a door, then you probably shouldn't be trying to defend your home with a gun anyways.
Something that hasn't been mentioned yet is that a shotgun doubles as a club. So if they have a knife, I've still got reach on my side. What's more, if you're assailant has the wherewithal to arm himself with a knife, the stealth to sneak up on you, the balls to confront a gun toting homeowner with only a blade, and the ability to get the gun off target, you have more problems than just firearm selection. Honestly, while it's a good worst case scenario, it might be the least likely of unlikely things that could happen.
It's no 10 round mag (fuck you california gun laws) but the 6 shell magazine should be MORE than enough to scare off or hit a threat... or 6. If you're chasing somebody around your house with your sight picture emptying a 15 shot mag in the process, you should probably reassess your firearms proficiency, and whether or not the law is going to be on your side. True, if they're coming at you or your family, I believe in keep shooting until you're empty, but 6 shotshells (even with bird shot) should be plenty to take down an attacker.
not to toot my own horn, but I can break three hand thrown clays at once with three shots and I can empty my 6+1 capacity in less than 2 seconds. How quickly do you want to pump out lead? It's certainly slower than a semi-auto, but REALISTICALLY how much firepower do you really need.
this is fair. If you are in a horrid situation, you're being invaded, zombies are breaking through your front door, the terrorists are attacking, or you're just making swiss cheese out of your house as fast as possible, reloading is going to take some time. With shotguns in combat, you reload every time you get the chance. While it is slower, the magazine CAN be reloaded without disarming the gun. With a shell in the chamber I can refill the tube without being helpless.
A lot of these points are aimed more at a pump shotgun then a semi. Just some food for thought. This is covered in more detail in Fernando Aguirre's book: "The Modern Survival Manual: Surviving the Economic Collapse"
In terms of combat, I don't think the shotgun is a catch all. I'd much rather have an AR or AK. But for home defense, where a missed shot means it could go into the next room where your family is, I like the shotgun. true, most shot (and all slugs) are going to go through walls, but with significantly less lethality than a bullet.
Whats more, most of us don't face combat on a regular basis. the shotgun's spread (while often WAY over-estimated) does help to compensate for trouble aiming due to panic. I can shoot skeet all day, but somebody breaks into my house, and no matter how well I've done at the gun range, I'm still going to be on edge.
It's not a perfect gun for everything, but it can do most thing pretty well, and I'd argue with proper training, it's particularly well suited to home defense. there's a video out there of an untrained woman firing nine shots with a .45 acp 1911, a .223 AR15, and a shotgun. Of the three guns she couldn't complete the task with the shotgun because the recoil was too uncomfortable, and the ar-15 was most accurate. So there is an argument for other firearms for home defense, and a good reason to make sure your gun fits you and your purposes. Better that somebody unload with a .22 and make every shot count than miss 10 times with something more powerful.
I love the 870, but if you can't make it work for you, it's worthless. Those were good points CorporateSlave, and certainly something to consider should you choose a scattergun for home defense or combat purposes.