Picatinny Rail
Also: Pic Rail · 1913 Rail
The standardized slotted mounting rail, spec MIL-STD-1913, used to attach scopes and accessories. Long-range builds use canted versions for extra elevation.
The Picatinny rail is a military-standard mounting base, MIL-STD-1913, with a row of evenly spaced cross slots and a fixed rail profile. Because the dimensions are standardized, scope rings and accessories from any maker clamp to it and index off the slots, so they return to the same place if removed and remounted.
For long range the important variant is the canted rail. A base machined with built-in cant, commonly 20 or 30 MOA, tips the scope downward relative to the bore, which hands back elevation turret travel for dialing distant targets. That extra travel is often the difference between reaching a far target and running out of adjustment.