A choke is the constriction in the barrel that controls how fast the shot charge will expand once it exits the muzzle; generally, they go from "full choke" (the tightest) to "cylinder" (meaning the interior of the barrel is just a straight-walled tube), but there are other specialized types that can do things like spin a slug, or spread the shot charge out in a horizontal line. These are useful in some sporting or gaming shooting, where you want a tight pattern out at extended ranges (putting a large percentage of the pattern on a clay target or game bird). Generally, they are seen on ALL shotguns, but the types you normally see on SxS shotguns are "built in", with the more open choke on the first barrel fired, and the tighter choke on the second barrel fired, and you can order whatever you'd like on most modern-built guns. So, depending on the shots you're expecting to take, you can either change the choke tubes out, or use a barrel set that is choked to deliver the best pattern for the range you'll be firing at.
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