Having been away from the Forum for several months, I realize how much I miss some folk's posts. Excaliber is one.
My two cents (and only MHO) on a couple of defensive shotgun points:
Choked barrels in anything tighter than police-cylinder are not a good idea. I'd stay with cylinder bore or police-cylinder. Tighter chokes are really intended for bird-shot only. Choking--logically--actually stretches many of the lead pellets as they come out of the barrel: if you take malleable lead pellets and smoosh 'em together at a constriction point under high pressure and a bit of heat, they're going to change shape. When you want a wide downrange spread to influence a duck in flight, that isn't a bad thing.
But what you want in a defensive shotgun is a consistent, uniform pattern density. When you change the shape of the pellets from round to ovoid, you get flyers, misshapen pellets that spin end over end and fly outside the mass of the main pellets. Not a good in a defensive weapon: you don't want most of the pellets to hit the bad guy, but a few fly off out of pattern and pop aunt Ellen and uncle Frank.
Similarly, while slugs can be fired through a choked barrel, accuracy at distance will suffer because, again, the constricted barrel will reshape the slug to at least some degree.
I also don't like rifled shotgun barrels. They can make your use of slugs more accurate, but it's far from a good idea for shot. What happens with shot in a rifled barrel is that, as the shot travels down the barrel and comes in contact with the lands and grooves, a spin is imparted both to individual pellets and to the mass of pellets as a hole. What you'll find if you pattern 00-buck out of a rifled barrel at 8, 15, and 25 yards is that the cluster of pellets expands more quickly than out of a smooth cylinder bore or police-cylinder. Moreover, the pellets are fanning out in what amounts to an expanding circle, not as a basically random mass of lead.
This is not a good thing. First, with an expanding circle of buckshot, as the distance to the target increases, the less likely you are hit what you aimed at. If you aim COM at, say, 20 yards, that expanding circle may miss the bad guy's chest, head, and shoulders completely, and the lower part of the circle may put a few pellets into his thighs. Second, those pellets that miss still go somewhere. Collateral damage downrange of the intended target is very likely.
What you really want is a predictable mass of buckshot out of smooth bore, with little or no choke, that expands--relatively slowly--in a consistent spread. A general formula is that 00-buck out of an 18"-20" cylinder bore will spread about two centimeters for every meter downrange, or about .7 inches per yard...give or take. So at 10 yards you still have a good, combat-sized spread of about 7 inches. That's controllable and effective.
You wanna hit what you aim at, not have pellets flying all over the place to locations you didn't aim.
Last up, I don't think it's a good idea to charge the tube with different types of rounds. Murphy's law. If the first round is birdshot, real life will give you a situation where you only have time for one shot and you need that first round to be the stopper. Similarly, if you load a few rounds of shot and then some slugs, real life will end up presenting you with a situation where you've fired your buckshot (or forgotten your round count!) but the use of slugs and the much greater penetration they afford would be risky to non-combatants.
I think the best solution is to keep one, tested load in the gun (I keep 00-buck, and wouldn't go smaller than number 1, even in an apartment), and have a few rifled slugs on side-saddle. Learning to perform an ammunition swap (essentially the same thing as a speed-load except you're first removing the chambered round and tube-fed round, if they're there) is pretty easy to do, but does require practice...just like anything else. Then if you ever find you need the penetration or greater effective distance of a slug, you open the chamber and manually load a slug.
