What is the difference between a cylinder and a improved cylinder bore ?
Cylinder Bore is no choke constriction at all. Improved Cylinder is about 1/4 choke. That is, if Full choke in a 12 gauge is about .040″ constriction, IC in a 12 is about a quarter of that, or .010″ constriction. In performance, an IC choke should shoot about a 50% pattern (half the shot inside a 30″ circle, half outside at 40 yards).
Shotgun Report’s Technoid