that know more than me.  What civilian AR15 manufacturer offers a rifle that matches as closely as possible the M16A2 that the US Army uses?

TY.  I was thinking more in terms of matching barrel length, handguard length and buttstock length, so the rifle will behave largely the same and fit me the same way as my issue rifle did.  The bayonet lug doesn't interest me much, but the birdcage does.  Does the pistol grip and a birdcage still a preban make?

Yes, pistol grip AND birdcage = preban  (unless you want to weld a magazine in place and cut a hole in the top of the upper to insert ammo [:D] ).  Of course you can have Kurt or Wilson Combat attact a muzzle brake that looks A LOT like an A2 birdcage to a postban, and get the same "look" without having to pony up the preban dollars.

I always hear talk about how Bushmaster is closest to Milspec out of the "Big Three" (Armalite. Bushmaster and Colt).

To get as close as possible to an issue M16A2 without the Class 3, I'd say go preban - otherwise I'd recommend Bushmaster just because they are the best overall value for money, quality and customer support.

Of course I wasn't in the US military, but from my experience in another military, I really wouldn't worry about it too much, if you are going preban.  Generally, there is so much swapping of parts and abuse in the military, and different configurations and variants of the same rifle - that any of the "Big Three" will sell you a "better" rifle than the average service rifle.

Thank you.