Heller '08 and McDonald '10 recognized self-defense as a justification for RKBA; it is high time that the gun control act be revised to recognize firearms for all lawful purposes, not just "sporting purposes".
My recap on this (and I welcome correction if I got anything wrong) is, best I can figure:
o The 4473 used to have three choices: Handgun, Long Gun or Both.
o August 2008 the 4473 was revised by ATF to have three choices: Handgun, Long Gun or Other.
o ATF instructions for 4473 in a newsletter for FFLs defined "other" as firearms that are neither handguns nor long guns (PGO shotguns, frames, receivers, lowers)
This new "other" category are Title I Gun Control Act "firearms" that are not Title II National Firearms Act "firearms".
Len Savage, Historic Arms LLC, 27 May 2010, sent a request for clarification to ATF Firearms Technology Branch. The answer they gave him was an "Other" PGO shotgun must meet the 26" overall length, but since it does not meet the definition of shotgun (long gun designed to be fired from the shoulder) an "Other" PGO shotgun with barrel under 18" is neither a SBS nor an AOW: as long as overall length is not concealable (under 26") it is still an "Other" PGO shotgun.
Shotgun with Pistol Grip: If it was sold on the 4473 as a long gun, you must observe 26" overall minimum and 18" barrel overall minimum.
factory original Pistol Grip Only Shotgun never had a butt: if it was sold on the 4473 as an "Other", you must observe the 26" overall minimum and may disregard the 18" barrel minumum, according to the ATF FTB letter in response to the Historic Arms LLC request.
My impression is that if you install a buttstock at sometime and the PGO shotgun barrel is under 18", then you would have to file a Form 1 for a SBS.
My further impression is that I ain't takin' no chances based on confusing instruction in a ATF newsletter and one ATF FTB letter on a confused subject.