The FAL Custom is a Black Market gun in CF Japan. Its power and accuracy are nearly equal to the normal M4A1 - it reloads slower than the M4A1, but comes with the optical zoom available to use. However, the grenade launcher is not functional like its Camo counterpart in Hero Mode and the firing rate decreases while zooming.
In Hero Mode, this is given to the Hero. This is called "FAL Camo" in-game, although there's no existing FAL Custom released in many CF Versions yet.
The FAL Custom is only available in the Japanese Crossfire. It was released with the Assault Girls Capsule Pack, which also includes the PPK and RAI Model 500.