The volume of actuating gas is adjustable. Behind the front sight is a small hole in the front upper handguard. This is a gas exhaust port and the opening is adjusted by turning the gas adjusting sleeve located on the barrel below the handguard. A dedicated spanner tool is available for this sleeve but, if the gun is clean, the sleeve can be adjusted by hand.
Tune the gas actuation so the bolt cycles well and cases are ejected. Too much gas and the bolt is cycled violently. My FN was misadjusted and the stock behind the receiver is cracked. Gas adjustment means the rifle may be tuned to the cartridge loading.