The bore of a 12 gauge shotgun is 0.729 inches (approx).  Cylinder bore (also called an "open choke") means that the bore is 0.729 inches from the chamber to the muzzle.  There is no restriction (or choke) to cause the shot to keep together longer after the muzzle.  This means that the buckshot or birdshot will start to spread as soon as it leaves the muzzle (and this means that the chance of hitting decreases as range increases).

 

The "choke" is a restriction in the bore that keeps the shot column together longer after it leaves the barrel before it starts to spread.  This means that the chance of hitting increases as range increases because the shot does not start to spread out as soon.

 

Buckshot or birdshot depends on a shot pattern at a given range to hit targets.  Because the shot spreads out, it is important that a certain percentage of the shot (total) remain within a 30 inch circle at a given distance.  This is called shot density.  The more holes in the paper within the 30 inch circle at a given range means that there is a higher chance that a target within this 30 inch circle will be hit. 

 

When buckshot is used it's for serious social purposes.  Engagement ranges are necessarily short because the cylinder bore allows the shot to start spreading at the muzzle.  Multiple hits are more likely to take down the bad guy rather than one or two.  A shotgun slug is one heck of a stopper -- if you hit your target -- and there's only one shot at a time per each cycle of the action.  With buckshot (or birdshot) there's a lot more lead out there to hit something.  From the standpoint of a social encounter, the mass of the shot matters.  Number 4 buckshot is the minimum considered (it's .25 caliber) and 00 buckshot (it's .33 caliber) are the most common used.  There are other buckshot sizes, but these are probably the most common.  The police and military use 00 buckshot in their combat shotguns.