FCU
Fire Control Unit — the serialized internal trigger group of a modular pistol; legally the firearm.
Also: fire control unit, FCG, trigger group
The FCU (Fire Control Unit) is the chassis-and-trigger-group module of a modular pistol like the Sig Sauer P320. It carries the serial number — making it, legally, the firearm — while the grip module, slide, and barrel are unregulated parts. This swap-the-gun-by-swapping-the-grip design is novel and has reshaped how pistol customization works.