I'm currently building an M16A1 and I understand all of the profile and design differences from current M16s especially with a great site like retroblackrifle.com, but I'm having trouble finding definitive info on later generations. I would also like to build clones of improved generations up to the current M16A4. Here's what I understand so far and I hope some of you can add or confirm this info:

M16 - M16A1:
• see retroblackrifle.com

M16A2:
• A2 grip adds finger notch and more texture
• A2 stock is 5/8" longer with stronger improved Zytel polymer and fully textured buttplate
• thicker barrel twist changed to 1:7
• elevation adjustable rear sight and square front sight post with four adjustment notches vs round post with five notches on A1 and earlier
• case deflector added to upper receiver
• closed bottom flash hider
• round hand guards with slanted retainer ring
• A2 lower receiver added radius reinforcement at pivot pin area and reinforcement around the receiver extension ring (are these any different from current generation lowers?)

M16A3:
• full auto fire vs burst of M16A2 (if that's the only difference, I won't be building one of these since I live in Kommiefornia.)

M16A4:
• changed to flat top Picatinny rail upper with removable handle
• Knight's Armament Company M5 RAS hand guard

I know this list is basic, so please help me fill in the blanks if any. I'm especially interested in learning if there are other lower/upper profile changes from current receivers.

A2 rear sight with elevation adjustment
A2 square front sight post with four adjustment notches vice round post with five notches on A1 and earlier
A2 added radius reinforcement at pivot pin area and reinforcement around the receiver extension ring.

A1, A2, A3 and A4 are all select-fire - A3 has AUTO instead of BURST 3rd position.  Current TM now classifies A3 as a flat-top, though you are correct it was just a full auto A2 when it was first introduced.  Now it's a full auto A4.

A2, A3, A4 uppers all have round bolt assist buttons instead of A1's oval.

A2  has the little glove box in the stock

How small are your hands?  

A1 had that too.

A2+(unless an updated A1) has the reenforced lower.

Thanks, guys. I've updated the 1st post. Let me know if anyone thinks of anything else.

What about the M16?  We had an all original one at Eielson. Armalite marked Colt receiver, Edgewater buffer, triangle charging handle, green furniture, etc.

The NCOIC of the CATM shop had to submit a waiver to prevent it from being destroyed.  AFAIK, it's the only one still in the military's inventory.

I guess I should have mentioned I was focusing on the "improved" models. There are a number of models before the A1 improvements which retroblackrifle.com covers very well already. Not being prejudiced against the pre-M16A1 models, but just wanted to fill the info gap as I didn't find any sites dedicated to post-M16A1 models.