Objective Bell
The flared front housing of a riflescope that holds the objective lens, whose diameter affects light gathering and how high the scope must mount.
The objective bell is the widened front section of a scope body that surrounds and protects the objective lens at the muzzle end. Its diameter follows the size of that lens, so a scope built around a 56mm objective wears a noticeably larger bell than one built around a 40mm objective. That extra glass gathers more light, which is the main reason shooters accept the added bulk and weight of a big bell.
The size of the bell has a direct mounting consequence, because a larger housing must sit higher to clear the barrel and any iron sights. Taller scope rings raise the line of sight and increase the sight height, a number that the ballistic solver needs in order to model the gap between bore and optic correctly.