I don't claim to be a resident expert, but I'll take a shot at answering your questions. I'm sure someone wiser than me will set me straight where needed.
1. Shotguns have chambers. Look through the bore of one from the breech and you'll see it. Yes, it is larger than the rest of the barrel. Theoretically, when the shotgun shell shoots open, the barrel diameter is the same as the inside diameter of the shell. The chamber is the outside diameter of the shell. Its an occasional cleaning problem, too. Some folks clean their bore carefully but ignore the chamber, which then becomes gunky and can affect feeding or extraction (after several seasons).
2. Chokes basically reduce the diameter of the barrel for the last few inches. Like the nozzle on a hoze, they affect the dispersion of the pellets. You can shoot slugs through a choke because the slugs are soft enough that they squeeze through. The constriction is pretty small. It is generally understood that a more open choke (improved cyl or cylinder bore/no choke) will give better accuracy with slugs than a modified or full choke.
Same can be true for buckshot. My shotgun shoots tighter patterns with buckshot using a modified choke than with either a full choke or an improved cyl. I guess the full choke squeezes the pellets too much, and they deform and fly wild. That's why its important to shoot paper with the loads you are considering and see what works best with your shotgun.
