The legal dilemma posed by pistol-gripped shotguns
Interesting issue ... and here I'm referring to shotguns which were originally made with a rear pistol grip in place of a buttstock.
Under the Gun Control Act, the issue seems simple: they are pistols. Very big pistols, but pistols. Thus they have been treated, and sold in large numbers over quite a few years.
The National Firearms Act, however, poses a problem. Under it, they are not shotguns, since the definition of "shotgun" includes "designed to be fired from the shoulder." 26 U.S.C. §5845(f).
The problem is posed by a different part of the NFA, created when it was amended in 1968. Those amendments created a class of firearms designated "destructive devices," and really aimed at artillery pieces. As you might expect, these were VERY tightly regulated, nevermind that criminal use of artillery wasn't exactly a big problem.
Under the NFA, "The term "destructive device" means .. any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary finds is generally recognized as particularly suitable for sporting purposes..." 26 U.S.C.§5845(d).
Every shotgun but the diminutive .410 has a bore greater than half an inch. The "sporting purpose" exemption doesn't apply, since that is limited to shotguns, and as we have seen, these firearms are not "shotguns" under the definitions of the NFA.
The situation as it stands is thus: BATF must never have realized that these firearms are NFA "destructive devices," and as a result thousands, if not millions, are in private possession of persons who bought them legitimately over the counter, with no reason to suspect that there was anything legally amiss.