I was just reading about Operational Briefcases and Secret Service style Uzi briefcases in Small Arms Review (July, 1999). Here's a portion of the article:
"There are other legal carrying devices for Title One weapons that are designed to both hide the weaon and to allow very quick access to it. These are legal because the weapon can not be fired from within . It must be removed first... Two good examples of these devices are the "executive planner" holsters and "Secret Service style" Uzi briefcases.
An Operational Briefcase also allows its user to fire the weapon while it is still hidden... The owner of a registered machinegun may legally possess an Operational Briefcase for it. Not even a notification to the BATF is required before buying one. The machinegun's BATF registration makes both items legal.
A non-machinegun owner may legally own an Operational Briefcase too, as long as he does not own a Title One firearm (handgun, rifle, or shotgun) to fit it. The BATF has ruled that the simultanious possession of both items, even though the Title One firearm is never placed into the Operational Briefcase, constitutes the possession of an untaxed, unregistered Any Other Weapon (AOW)."