OP, I had a friend's son buy an AR like the one you have from Bubba, at a gun show last year. They bring it over to me all excited about it. Right away
I notice it had holes in the mag well where it clearly used to be a 9mm lower but it had a Hbar 5.56 upper and barrel on it. Then I noticed a hole in the receiver drilled for an auto sear. It still had the full auto fire control group in it too. He was on his way to the range when he stopped by. Thank God he didn't make it there. All in all he spent $900 for a gun he could only use the upper, receiver extension tube, buffer, spring, stock, and sling. The lower and fire control parts had to be disposed of. It made me sick but that is what had to be done. It was a Colt too. Long story short I hope your not in the same position my friend and his son were. Make sure it isn't stolen and make sure the receiver and fire control group are legal. Get a pinned flash hider on the barrel to make it 16 inches or a tad longer. Its a damned shame that the bastard that sold it to them didn't get busted for trying to sell it and spend some time behind bars.