I'd imagine it falls under the commerce clause. Remember, the drug war and countless other prohibitions is based on the fact that the government can arrest you for something you grow in your house and consume only yourself, because that affects interstate commerce since you might have otherwise bought it from a source that might cross state lines. Given that expansive interpretation, which the courts have upheld every time except one, how is everything that happens on a plane actually crossing state lines not fair game? And you might think this is wrong and support pot legalization, but it's also equally applicable to cocaine and heroin. Which come from plants, by the way. Plants which I can grow entirely on my land, extract alkaloids, sell to no one, and be thrown in prison for the rest of my life for.
But guns on the other hand... perhaps someone can help me reconcile US v. Lopez with the constitutionality of the CSA as applied to wholly intrastate actions.