Apropos Orin's post below, the effect of the right to bear arms on sentencing enhancements has already come up under some state constitutions. Forty state constitutions secure an individual right to bear arms. (Six don't have a right-to-bear-arms provision, two provisions have been interpreted as securing only a collective right, and two more are textually ambiguous and have not been interpreted by their state courts.) Some state courts in those states have considered whether enhancing a sentence based on possession of guns violates the state right to bear arms.

The general thrust of those decisions that I've found is that such enhancements are constitutional if the government proves a connection (usually called a "nexus") between the possession of the gun and the crime, but unconstitutional otherwise. See, e.g., State v. Eckenrode, 150 P.3d 1116 (Wash. 2007); People v. Atencio, 878 P.2d 147, 150 (Colo. App. 1994); see also Brewer v. Commonwealth, 206 S.W.3d 343, 347 (Ky. 2006) (requiring a nexus between a gun and the crime before a gun can be confiscated in a forfeiture proceeding). How close the connection must be, I can't tell you, though if you're really interested you can read the cases to see what bearing they have on that.

I also can't tell you for sure whether federal courts would accept this test, if they recognize an individual right to bear arms. But it seems to me to make sense.

On one hand, the right to keep and bear arms does not include the right to use them in crime, whether by firing them, brandishing them, or even having them available so they can be used (and thus so they can embolden the criminal and his associates). To the extent the sentencing enhancements require a finding that the gun was possessed in furtherance of a crime, as Orin says they do at least under one statute, that should be constitutional.

On the other hand, the mere possession of a gun at the time a crime is being committed, even if the gun is far away or if the crime is not at all furthered by the gun's presence (say, it's the distribution of online pornography), likely can't constitutionally trigger an enhancement: That would simply be punishing someone for constitutionally protected behavior (possession of a gun, not use of a gun), which would be impermissible even if the person was also engaged in some properly punishable behavior.

Incidentally, something similar arises in the use of membership in a racist organization as a sentencing factor, where the First Amendment right of association is potentially involved; see Dawson v. Delaware (1992).