You need to SBR those babies! Earlier this year, I bought an M-16, from my XBIL. It turns out it was an expermental gun. His brother picked it up on the battle field in Nam and brought it back under the military grace period. I did the tax stamp and transfer and then put a 7 1/2 inch upper on it in 5.56. Just purchased a 10 inch upper for it in 6.8 from LWRC.
Bill