Gun ownership is not illegal, just difficult.
From Wiki...
Dutch gun law is typical of the Western European approach. Firearm possession is not subject to any constitutional protections, but regulated simply in the Arms and Ammunition Act (Wet Wapens en Munitie). Weapons, including firearms, are divided into four categories, and for each of the categories a certain maximum punishment is set for "voorhanden hebben" (possession), and "dragen" (carrying in public).
Only citizens who are hunters, members of shooting sports clubs or legitimate collectors may obtain licenses for firearms. In the case of shooting club members they will get a license for category III weapons (a firearms category which includes all non- full automatic firearms up to .50 cal.). Possession is generally limited to 5 firearms per license. Collectors may obtain a license for any category of firearm, including full automatic arms, but stringent rules apply to achieving collector status. There is no limit to the number of firearms a collector may obtain. Generally the collector license does not cover shooting these weapons.
Sale/Use is only for those age 18 or over. There are a few dozen gunshops in the Netherlands. Gun ownership is extremely low with only three firearms per hundred people.
Antique firearms are exempt from regulation, antique generally meaning all pre 1945 original muzzle loading firearms or firearms designed for black powder cartridges over .22 caliber, NOT being revolvers.