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.
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