This area is a bit complicated. A pistol, to be a "pistol, under federal law, must also have a handle ("short stock") set at an angle to the barrel, and below the barrel. Therefore, the law currently does not define a pen gun as an NFA weapon because the gun is hinged, and the hinged portion is designed to be bent away from the barrel before it can fire. Thus, the hinged portion becomes the "Handle" which is set below, and at an angle to the barrel. In April 2005 when ATF applied to change the defination of "pistol" to eliminate pen guns, wallet guns, belt buckle guns, and pager guns, the chnage would affect 27 CFR 479.11, but to date - nothing's happened. Assuming your once legal firearm suddenly becomes an NFA weapon, you will have a grace period to register it as "any other weapon". So, for now, you're OK. To make it simple, if you have any weapon that discharges a shell by use of an explosive, the weapon can be concealed on your person, and that weapon does not hve the normal configuration of a pistol or revolver - it probably is "any other weapon" under federal law. Similiarly, a derringer that can be fired from within a concealing holster becomes "any other weapon" only when placed in such a holster, (wallet Gun), because the angled grip extending from the barrel is no longer visible. Same thing with a gun rigged to be discharged from inside a briefcase. As long as the handle is set at an angle slightly below the barrel. Even that slight angled grip to the barrel brought it within the definition of a pistol, according an ATF ruling. However, if the definition ever changes - it"s "any other weapon". So, if you have a derringer with a special wallet holster, are you illegal? Sort of! Once you put the two together ATF says you consider then as a "unit". The unit has now lost the "angled grip" it should have to be a pistol. You have a potential ten year federal felony! 26USC 5871. Hope this helps. It is a lot to digest and it is not worth the hassel.