The primary differences between the A1 and A2 models are as follows, the rear sight(and reciever), the barrel contour, the rifling twist, the handguards, the stock(and length of pull), the pistol grip, the fire control system, and the flash hider.
There are several lesser differences, the lower reciever has reinforcements in key areas now, and a few dimensions have been changed, but that's the gist of it. IMHO, most of these changes have been for the better.
However, I think that the M16A1 was the better infantry weapon for several reasons, the biggest one being the rifling twist. The 1/7 twist of the A2 is great if you buy the current military thinking that wounding an enemy is preferable to killing him, as the heavier 62gr FMJ when stabilized at this higher twist makes for better penetration and less bullet tumbling.
However, bullet tumbling in the 1/12 twist A1 was what gave it the great wounding capabilities noted in Vietnam, and I think that outweighs any advantages gained with the heavier ammo. The other big bitch I have with the A2 is in the fire control system. (rant mode on)
I don't know who the pencil pushing, bean-counting REMF was that came up with the idea of the 3-round burst, but one thing I do know, he wasn't infantry. If you need full auto fire, you NEED FULL AUTO FIRE, and that's all there is to it. Training is what keeps the bursts short, or keeps the grunts on semi and squeezing them off. There might be places for cost-cutting measures in the military, but the M16 is not the place for them. (rant mode off)
The barrel and sights are debatable, I like the lighter weight and simpler system of the A1, but many do like the steadier feel and improved sights of the A2. I do like the larger 0-2 aperture of the A2 sight for close work and low light, and it wouldn't have been much work to install an A2 aperture in an A1 reciever and save about half a pound. My AR, in fact, is set up this way.
The stock, handguards, and grip are all better, I think, as I'm pretty tall and they fit me better. These are pretty much personal preference, except for the handguard, which is two identical pieces instead of two different ones(simplifies support). I have a sentimental preference for the triangular A1 handguards.
I guess this pretty well covers it, my e-mail's on here if you want to talk direct. Hope this helps.
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My brain's a hand grenade...catch. Ice-T, OG.
The M-16 is a damn fine weapon, but your best, your most lethal and effective weapon is between your ears. Ssgt Brown, Parris Island, 3rd Bn, H co. 1984.
