the.com/full metal jacket

means A bullet with a hard metal shell (usually copper or a copper alloy) wrapped around a soft lead core, designed to hold its shape on impact instead of mushrooming.

from From late-1800s military small-arms design: 'jacket' had long meant an outer covering, and arms-makers borrowed it for the metal sheath encasing a lead bullet — 'full' metal jacket meaning the casing wraps the whole bullet, nose to base, rather than leaving lead exposed at the tip. The design itself emerged alongside smokeless powder and higher-velocity rifles, which made plain lead bullets prone to stripping or deforming in the barrel; cladding them in a tougher metal solved that. The phrase later became widely known to civilians through Stanley Kubrick's 1987 Vietnam War film 'Full Metal Jacket,' which borrowed the ammunition term as its title.