Taken directly from NWA/ KLM web site.
Guns, Firearms
Carry-on:
NW/KLM does not accept guns or firearms of any kind in carry-on luggage.
Checked: NWA accepts handguns/BB guns/rifle/shotgun type firearms only as checked luggage, with certain limitations and requirements. KLM accepts sports guns only, handguns are not accepted. Passenger must obtain and comply with regulations governing the transportation of firearms for all countries on the itinerary. All firearms must be in a suitable container/crush proof container manufactured specifically for firearms or in a hard-sided suitcase. Passenger must verbally declare firearm is unloaded. NW requires firearm unloaded tag be signed by passenger. Tag must be placed inside of luggage containing firearm. No exterior tag nor notice of firearm may appear on case. Case must be locked and only the passenger may retain the key or combination. NW/KLM personnel will not unload or handle firearms. Gun and ammunition may be in the same piece of luggage. However, ammunition must be within its own packaging which should be in factory carton or rigid shock proof shipping container. Note: gun powder is not permitted as checked luggage including propellant charges for muzzle loading firearms such as pyrodex, etc. Maximum ammunition per passenger: 11 pounds or 5 kilograms.
International shipment of arms/ammunition:
Passengers must obtain and comply with regulations governing the transportation of firearms for all countries on the itinerary. Passengers should check with Northwest or KLM for current policies, requirements and restrictions on firearms and related items in checked luggage when traveling in the U.S. or internationally.