Bottom Metal
The underside hardware, the trigger guard plus magazine well or floor plate, that ties the action into the stock.
Bottom metal is the assembly fitted to the underside of the rifle, combining the trigger guard with either a magazine well or a hinged floor-plate that retains the rounds. It clamps the action down into the stock and provides the opening through which cartridges feed, so its fit and rigidity matter to how consistently the barreled action sits in the rifle.
Builders choose bottom metal to match their feeding setup. A blind or floor-plate unit suits an internal magazine, while bottom metal cut for a detachable-box-magazine accepts removable mags that drop free for fast reloads, a common upgrade on precision rifles.