Scope
Also: Riflescope · Telescopic Sight
An optical sight with magnification and a reticle, mounted above the bore. The single biggest contributor to long-range accuracy after the shooter.
A precision scope has three things going for it: enough elevation travel to reach the target distance you intend to shoot, optical quality that holds up at high magnification, and turret clicks that move where they say they move (repeatable tracking).
Cheap glass gives all three away. A $1,500 scope on a $3,000 rifle outperforms the reverse arrangement in every test that matters.