How about this bad boy.
http://www.dixiegunworks.com/popup_image.php?pID=14019
About 18" long, .50 caliber, essentially a sawed off shotgun, and perfectly legal in the State of Washington without any paperwork required. Originally used in India for close-in protection against tigers back in 1890 or so.
A rifle strikes me as a fantastically stupid choice for self defense in a home. Powerful long gun bullets will goi through walls, so it’s a hazard to people in your home you DON’T want to shoot - indeed, in many calibres it’ll be a hzard to people in other homes. Additionally, a rifle or carbine is relatively unwieldly when moving inside a home. A handgun is a far better option, unless you have one hell of a big home. It’s not going to get caught on stuff, and you don’t need to shoot someone 300 metresa away.
Two barrels.
Cool!
what about it? it’s a muzzleloading black powder piece.
That’s right, and I sure wouldn’t want it going off in my direction from a range of ten feet or so!
black powder is unstable and hygroscopic. I wouldn’t want to rely on it if it’s been sitting around loaded for any length of time.
As noted by others, a shotgun is usually the best weapon for home defense. But an argument could be made that your weapon of choice should be the weapon you are most familiar with. I am most familiar with my FAL, and thus it is my go-to weapon for uninvited guest.
I know it is supposed to be unreliable after storage for any length of time. However, in the days of wooden ships and iron men, the black powder for the guns was stored in wooden casks in a magazine that was usually below the water line, in an environment that was of the highest humidity, and those ships were out on patrol for months or more at a time.
I have never heard of an instance where a battle had to be called off because the powder wouldn’t go bang.
knife, dark, home court advantage
To answer the OP, I think my optimal choice would be an AA-12 Combat Shotgun with a 20-round drum, and several extra drums of ammo scattered around the premises for a quick reload if necessary. Good against one intruder, good against a dozen. Ideally I will face the intruders somewhere away from where my most precious stuff is, so nothing I really care about gets destroyed in the hail of shotgun pellets.
And as far as this, while that may be somewhat true for people out in wilderness locations, if armed rebellion against the government ever became a reality, it would be in cities with urban warfare. AR-15’s are probably reasonably useful in the urban warfare that would realistically take place in such a ridiculously unrealistic occurrence.
I have an assisted open every day carry knife as a tool, but I wouldn’t bring it to a gunfight.
Keep a good lawyer handy. I (truly) used to keep a pair of .69 smoothbore belt pistols loaded with a mix of buck cast onto piano wire and swan shot near the nightstand. Stick my arm around the stairwell and anything on the first floor was going to have a bad night. Using something that far outside factory specs could “show a clear intent to maximize lethality” and get you in front of 12 folks no matter how clean the shoot. Modern factory stuff flies past the grand jury easier.
50 is a little light with shot and those are rifled tossing your spread all over the place. But I do like the way you think. 
Not really if you know it/love it/live it. For hunter ed classes I like to have a misfire so I can show the kids how to handle one. Having done things “right” so long and having well tuned guns, I had to resort to not priming at all or jamming a toothpick in the touch-hole.
Plus they have these things now called “powder pellets” --------- cool invention.
I picked the UZI. I have actually fired one, full auto, and was able to put rounds on target from the first burst. No one else lives here so no worries about stray rounds. Plus it’d be cool to have in general.
Poor choice. I’ve been shot with #6 out of a 12 gauge and it didn’t break the skin - just left welts. I think it CAN break the skin, but at best you’re shooting to wound. Which is terrible because it relies on the guy deciding to flee. He may well just get pissed off. I did. And even if you blind him, now he can’t flee, and is enraged. Go with 00 minimum.
I should have given more details. The 50 caliber version is rifled, and is mainly for round balls of that caliber. There is a smoothbore version that is .625" diameter.
My weapon is an M1 Carbine. Plenty of bullets and fast rate of fire, not excessively loud, adequate stopping power. Cool to look at for it’s historical value even if I never have to fire it in anger.
The issue is range. While bird shot will not be as instantly leathal as buckshot, it certainly can and will kill or disable at close range - and in home invasions, close range is what you will have.
Buckshot is much better in terms of lethality, but birdshot will work.
.22 will work too, but that doesn’t make it a good choice. If the intruder is armed with anything more than a spork, shooting to wound (with a chance of potential lethality) is a bad idea.
I don’t like excessive bio-mess so I don’t like dogs and I don’t favor eviscerating someone with a 12ga. I also want to minimize the need for drywall repair and collateral damage to the kids and cats.
First option is the window if it’s just me and the missus. If the kids are home I’d need to go a’hunting with a .22 pistol. The way I see it, IF I can get a decent shot off it’s going to hit or miss whether it’s a .22 or .45 bullet coming out. More than likely, I’m gonna miss so I want to minimize the consequences of that, and I want to reacquire my target for another shot. Hard to do that (for me) when I’ve got a massive, fire-breathing hand cannon–casting daze and stun spells at me–flying all around my head. I don’t believe in deterrence with a big, nickel-plated bazooka. If things are really so bad that I need a gun, it’s coming out and somebody’s gonna get shot. I’ll be damned if I’m gonna show that card and let an adversary have a second or two to work out his next move.
Your strategy depends if you have other people in the house you want to protect and the layout of the house, but I figure the best way is to lay prone or crouched at a corner to present the slimmest target and wait for the intruder to come into view. This should give you 1-2 seconds of lead time since you’re already aiming a the corner, you can hear him coming, and it will take him a bit to even see and process you. For that it seems like a pistol would be much preferred due to the kick, but I dunno.