According to this Annotated 2nd Ammendment link, US v. Miller upheld the registration of Short Barreled Shotguns because they were not "any part of the ordinary military equipment or that its use could contribute to the common defense."

By the same argument, however, SBRs (specifically 14.5" barreled M4s) should be legal without registration.

I'm sure this argument (and even the same argument for machine guns) has been raised before, but it was just a passing thought....