I'm not saying the way e-books work is good - it clearly isn't - but not because there's a fundamental law that owning an e-book should be identical to owning a physical book.
In your examples, you own the book and the gun because the publisher of the book and the manufacturer of the gun (or anyone else, excepting the state, I suppose) can’t unilaterally take them back from you or modify them without some sort of penalty. This can — and has [0] — happened with eBooks. The “unlimited rights to do what you want with [them]” are not guaranteed by ownership in my opinion.
[0] https://www.nytimes.com/2009/07/18/technology/companies/18am...
Not so for a DRM’d ebook.
Eminent domain is the state taking from you. I already mentioned this in my comment. In fact, the state pays a penalty for doing so in the US as enshrined in the 5th amendment: “… nor shall private property be taken for public use, without just compensation.“
> Ownership does not have a single neat list of requirements
No, it does not have a comprehensive list for all use cases, but I would say that the one I mentioned in my first comment is a necessary one: fundamentally that it cannot be taken from you without penalty.
Copying falls under the ownership of the text within the book. It is something completely separate.