I think a semi-auto carbine is an excellent home defense weapon. Many others indicate a shotgun would be better. I think a shotgun is fine for someone who is extremely adept with it. It takes a lot more practice to become adept with one, I think, than it does to with a carbine. A shotgun is harder to aim if using slugs, not as hard with buck shot. It kicks like a mule if using a 12 gauge. It is a powerhouse with which to shoot someone. A great gun yet, I would not want to miss with one for sure. I like them a lot but maybe a carbine could be better for home defense - I am in flux over this lately.
A well chosen home defense pistol caliber, coming out of a carbine, is probably less likely to penetrate walls than would be a shotgun slug and maybe even than would be 00 buckshot - a big consideration depending on what or whom is on the other side of the wall. Of course the pistol round would probably penetrate more than pepper shot. Whatever you decide on ammo, I still think the carbine would be a better choice for most shooters. Shotguns are much more difficult to wield in combat situations than are carbines. You can also fire much faster with a carbine for follow up shots and would probably be better able to maintain accuracy under stress than with a shotgun.
And a semi-auto version is only good for the initial shock value
As for this, it makes me laugh. I speak from years of experience as a firearms instructor, specifically as an MP5 shooter and instructor. Granted I have not fired full auto for years, but I have done it. It was not spray and pray once you knew how to do it. In more recent years my job only used 3 round burst - semi-auto versions of the MP5 but I can tell you that when firing it in single fire mode (semi-auto) I could empty three full magazines, at multiple targets, faster than everyone I ever trained while they fired three round bursts. I did it with a good deal of accuracy too even though I had to change trigger fingers (index, to middle, back to index or to ring finger) once or twice before completing three mags of firing. Please tell me about the weapons use only for shock value in semi auto mode.
Of course a pistol is also a good alternative, with less chance of over penetration than a carbine shooting the same ammunition but, I think in many situations you will find more control with a carbine.
Weapon retention is also an issue with a long gun in close quarters.
As for people who believe that weapon retention is a factor to consider, I could not agree more but probably for a reason other than was thought of by the person who said the above. Therefore I would have to say - go with a carbine if you are concerned with weapon retention being a problem. While it may be a bit harder to grab onto a pistol someone is holding - it is definitely easier, MUCH easier, to do a weapon takeaway with a pistol than it is with a carbine. In other words, I believe you have a much greater chance of holding onto a carbine that someone tries to take from you than you do a pistol, that is once the bad guy has grabbed hold of your firearm. I also think a carbine is less likely to be jammed during a takeaway/retention attempt than would be a pistol - this can go for or against you depending on whether or not you retain the weapon. You are also more likely, in my opinion, to be able to control the direction in which the muzzle is pointed in such a confrontation if you are holding a carbine rather than you would be the muzzle of a pistol. If you are in doubt about any of this, just take hold of a red gun pistol (training gun that cannot possibly load or fire ammunition) and ask someone to take it from you - allow them to calmly grab hold of it, then on a count of 3 try to retain it as they try to take it from you. Do it a few times so each of you gets the hang of it. See who winds up being the winner as time goes on and the number of tries goes up. Then ask someone who knows how to do a gun takeaway, like a cop or a felon. Use a pistol first, then try all over again with a red gun carbine. (Make sure your finger is outside the trigger guard during each takeaway attempt with either pistol or carbine or your finger may get broken.) When doing this make sure to hold both in combat ready stance. I think some of you are going to be very surprised at how easy it is to lose the pistol, and how much more difficult it is for an assailant to take away a carbine from you. Sure there are probably some great takeaway moves for a carbine that would make it easy for a well trained and practiced expert but; the two hand hold on the carbine at two different points, many inches apart, in my opinion, makes it much harder to steal the carbine away from your grip.
Now after having said all of that, I will admit my personal preference for a home defense gun used to be a Remington 870 12 gauge shotgun. If there were none of the above tactical considerations, like who is where in my home, reloading, and so on - it possibly still would be my choice> Then again I am very adept with it regarding tactical shooting. While I also shoot an MP5 excellently, I do not own one, it belongs to my job and I do not think they would appreciate my routinely keeping it loaded in my bedroom - ready for action. I do like my shotgun although when I have thought about it lately the carbine comes out better and better. If I owned a carbine, it might be one that would share the same type of magazine as did one of my pistols. This way if I needed to reload, I could use the same mags in either pistol or carbine - a real tactical advantage over a shotgun/pistol combo.
Instead of either though, practicality has made me choose another weapon. I have chosen a pistol (which I sometimes now keep unloaded at bedside - up until recently I usually did not keep a pistol handy when sleeping but certain events have changed my mind about this). Why the pistol and not a carbine. First of all I have the most practice and training with a pistol. Secondly, this is a practicality issue for me; I do not currently own a carbine. I cannot currently afford to buy one. I did have one a Marlin Camp 9 - I believe it a terrible choice for a combat or home defense weapon - but that would be an issue for another thread. Then there is yet an additional concern I did not mention above. Despite my thinking the carbine would be a better choice of home defense weapon once in hand, the thing is getting it in hand that can pose a problem. I can walk around my home with a pistol attached to me somewhere and not even realize I have it on; I cannot do that with a carbine. Yet if ready access to a ready carbine was not a concern, it might well be the weapon I would choose to have if an intruder broke into my home (tough choice between a carbine and an 870 but lately I am thinking more and more of a carbine especially with each resounding thump of the 870 against my shoulder).
A carbine definitely could be an excellent home defense weapon for you and for me. Of course for me, all that would depend upon my ownership of a carbine. I don't currently own one. I may just have to do something about that like start scrimping and saving. If only one had the luxury to always practice that in which they believed - anyone want to make a donation???