Scope Base
Also: Mount Base
The hardware that bolts to the receiver and gives the rings a standard surface to clamp, often built with built-in cant for long-range shooting.
The scope base is the foundation that bridges the rifle’s receiver and the scope-rings that hold the optic. It bolts to drilled and tapped holes on top of the action and presents a standardized profile, most commonly a picatinny-rail with its uniform cross slots, though the older weaver pattern is still found. A solid, properly torqued base is the first link in the chain that keeps a scope holding zero under recoil.
For long range, many bases are machined with a built-in forward tilt, frequently twenty minute-of-angle, so the scope points slightly downward relative to the bore. This cant shifts the optic’s internal travel toward elevation, letting the shooter dial far more drop before running into the scope-elevation-ceiling. The tradeoff is that an aggressively canted base can use up the down-travel needed to zero at close range, so the amount of cant should match the intended distances.