Bases

A base is the part that fails silently. Everything above it can be excellent, but if the base moves under recoil the rifle will not hold zero and the symptom looks exactly like a bad scope. It is the cheapest component in the stack and the one worth buying properly.

Bases are receiver-specific — a Remington 700 short action base does not fit a long action, and neither fits a Tikka. Picatinny is the modern standard and is what most rings expect. Bases with 20 MOA of built-in taper give a long-range scope more usable elevation, which matters past about 600 yards and is otherwise just a slightly odd zero.