You'd be amazed by what you find in Dutch museums
Many don't have a clue what they have, and since the Netherlands are not exaclty a pro-gun country, most don't care either.
An exception is the Dutch Army Museum, which was located in Delft, but will move to the former Soesterberg airport together with other military collections. They have a very nice, large, well documented collection.
The best material is in private collector's hands, not in museum collections.