So, what would happen if I took one of these:
…replaced its existing pistol grip with one that could accept an AR-15 buffer tube:
…inserted one of KAK’s pistol buffer tubes into it:
… and then wrapped a Sig SB15 Arm Brace around it:
My understanding of the BATFE compliance letter (IANAL and all that), the PLR-16 started out as a pistol, and neither attaching the AR-15 buffer tube nor the arm brace changed that, so one should still be in the clear. Furthermore, “constructive intent” only seems to apply when you have no other purpose for the parts you have on-hand except violating the laws (IANAL again), so the fact that I happen to have a spare carbine buffer tube and stock is irrelevant on account of my still having the AR15 they came off of (they were replaced with a Magpul UBR).
The question only comes up because my SB15 is winding its way towards me as we speak, as is one of KAK’s pistol buffer tubes, and I have been pricing out AR-type build options and prices once I have a steady stream of income again. It appears that purchasing and kitting out a PLR-16 is marginally cheaper than using the bare, never-built AR-15 lower I have in my safe as a basis for an AR pistol, plus the PLR would not have the stupid-assed buffer tube hanging off its back end if/when the BATFE finally recants their approval of the SB15. OTOH, it is a Kel-Tec, which can open up a whole ‘nother can of worms in and of itself.
Anywise, just idle speculation.