Well, if you've been in the Army, likely you will know the M-16A2 forwards and backwards and inside-out. You ALSO know that it is selective-fire: semi- and full-automatic.
And you also know that a full-auto is the next best thing to a one-way ticket to jail, unless you go through all the Class 3 stuff and lay out $15,000 or so. (Funny, the Army paid $109 a piece for them during the 'Nam war.)
So you need an AR-15 that looks like an M-16A2.
Colt is the military contractor and they do make a civvy model. It is very nice and it is expensive. Thing is, about 40 other places are all making them, too. Most of them get the receiver castings from the same contractor that makes them for Colt, Colt not doing their own castings. So a LOT depends on fit and finish and how much people CARED when they made them.
I would go with one of the 'name-brand' manufacturers for something like this, simply for ease of getting service just in case something does go kablooey on you.
So you look at original Colt, you look at Smith & Wesson and a couple of the others, you find out what the price-tag is, take your heart pills, lay out the money and go home.
At your local gun shop. What most of them don't already have, they are more than willing to bring in.