The FN SCAR-L used the common design of the FN SCAR family using massively polymer and aluminum components. As for the other members of the SCAR family, it features free-floating cold hammer-forged barrels with a chrome-lined bore. The FN SCAR-L is fitted with a gas regulator with two positions. The first position is for normal operation, while the second position ensures there is no increase in the cyclic rate of fire when using a sound suppressor. The FN SCAR-L assault rifle features an extended top MIL-STD 1913 Picatinny rail integrated to the monolithic aluminum receiver. Additional Picatinny rails are mounted on each side at the front of the weapon as well as under the barrel. The FN SCAR-L is fitted with an ambidextrous 3-position safety selector easily manipulated with the thumb of the firing hand. With the safety selector in the “S” position, the rifle is in the “on safe” position. With the safety selector in the “1” position, the rifle is ready to fire one shot. In the “A” position, the SCAR-L can fire in fully automatic mode.

The SCAR-L is designed to operate with NATO 5.56x45mm or .223 Rem caliber ammunition. It has a standard M16-type magazine with a capacity of 30 cartridges. The bolt/catch release locks the moving parts to the rearmost, open position after the last cartridge of the magazine has been fired. To release the moving parts when no magazine is in the rifle and the bolt/catch release is locked rearward, press inward on the bolt/catch release, or pull the charging handle to the rear to disengage the bolt/catch release and then release the charging handle.