From what I'm to discern, Beretta's Optima HP choke tubes are slightly tighter than other standardized choke tube diameters, so you're correct, but Beretta calls it Cylinder Bore. Beretta doesn't recommend using slugs through the Optima HP Modified tube as it too is tighter than a normative modified choke tube. I've shot both slug and buck (flight control) through both their Cylinder Bore (4 dashes) and their Modified and found the Modified was the proper/best choke tube. I'm confident the performance translates to birdshot.