What does a cylinder bore choke mean?
Basically, no choke at all
"Cylinder bore" means the bore diameter at the muzzle is the same size as the bore at the front of the cylinder where the shell is when it's fired. Shotguns use "choke" settings to restrict the size of the mass of lead pellets coming out of the muzzle to tighten the mass down onto a further target....a cylinder bore gun has zero choke restriction so the mass of lead pellets stays wide and spread out…
The Choke of a shotgun is the amount of barrel restriction at the muzzle end which affects the shot pattern. A full choke will keep the spray pattern of the pellets closer together farther out than if the barrel was not restricted at the muzzle. If there is no restriction at the muzzle, the choke is said to be cylinder choke or cylinder bore.