Assigning a single reason as to why the market values a particular shotgun the way it does is a fallacy. Weight may be one factor. There are likely lots of others. It's like when the talking heads on tv say the market was down on profit taking or up based on consumer confidence. It's almost never a single factor. And there are lots of things that go into collectibility. Just because something is rare does not make it collectible.

Nothing the government gives you is free.