[Sloth]

As others have said, the Tenth Amendment. The Second is not even necessary to address the question from a Federal standpoint -- the reality is that while the Federal government may not be explicitly forbidden to register guns, it is also never authorized to do so. The language of the Tenth makes it clear that Constitutional silence regarding any power means that power is prohibited to the Federal gov't.

The States are another issue, however.

[dirtboy]

As others have said, the Tenth Amendment.

Problem is, the Tenth is basically moribund. I don't like that, you don't like that, but SCOTUS says otherwise, and they tend to carry more weight on the subject than an FR debate. And, under current interpretation of the Commerce Clause (once again, I dont' agree with it), a commercial gun sale is subject to federal regulation, which could include ballistic fingerprinting. Notice that the gun-grabbers are initially talking only about this requirement for new guns being sold.