The law has clearly morphed, I think the reason for that is we have agencies changing regulations without clear legislatures voting on the changes, 41P for example. I first noted this after the shopping center shooting in SLC, trolley square. An 18 year old bought a pistol grip shotgun with a 30 inch barrel. The pawn dealer was charged and convicted of selling a handgun to an underage buyer.
Regarding the weapon of 26 inches of length being discussed, these have been around in the periphery for a couple of years, see shockwave technologies website. http://shockwavetechnologies.com/site/?page_id=88
I think one needs to be very careful about building one bought second hand or bought years in the past because one does not know how it was transfered from the dealer previously, if it was transfered as a shotgun, would guess that the accused would have the burden of proof that it was never in "shotgun" configuration, which the fed define as a gun with at least an 18 inch barrel and a buttstock.
Personally, I am going to wait to buy the non NFA version until they are a little more mainstream...I form one registered my mossberg because I figured $200 is cheaper than getting an attorney and going through the BS if I were ever arrested with one of these "PGO's" and I needed to prove that I was within the law.