YOU CAN'T MIX THE LOWERS @_@ so illegal.
I bought a lower and when I filled at the form I told my LGS I am doing a AR-pistol and they filled in the blank on the 4473 as a "pistol" lower NOT RIFLE! I recommend engraving it as "pistol"
I think legal lengh max you can put is 14" the 16" is for rifles. I seen 7, 10.5, 11.5 12.5 14" so as long as your lower is "pistol" you can put these on(check state laws). But if it was a "rifle" lower, you can't put those barrels under 16" on it unless it was SBR.
SHTF vs TEOTWAWKI you ask? SHTF, just some crap on the ceiling and walls, TEOTWAWKI end of the world as we know it. Orignally made by 9mm/®™©