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The Colt M16A2, a later step of the M16, was adopted in the 1980s, it has some improvements, and some "improvements".
Most visibly is the change in furniture, the original triangular handguards were made of a left and right half, they were somewhat brittle, and as it turns out, it was very common for just the left side to break. This was somewhat of a logistic annoyance, as it means you would end up with a lot of right side spares and have to get left side spares.
The new handguards are two halves and are identical, you just turn it around.
The pistolgrip now has a fingerstep on it, some people really hate this fingerstep because it doesn't fit their hands, I'm one of those people. It wasn't entirely unheard of for someone to grind and polish off the nub (which was probably against regulation).
The stock is the same style, but the manufacturing is slightly changed.
The rear sight is changed too, still a fixed carry handle, but now having a detachable sight unit fitting onto it, this rear sight is adjustable for windage but also elevation. Never cared for this sight, maybe good for target shooting, but I think the A1 style was enough for a combat rifle. I've heard it's very easy to accidentally 'un-zero' these sights by bumping them against something, and I've also heard that it's not true.
The lower receiver is reinforced with thicker material right behind the forward assist, where the stock/buffer begins. This was probably not necessary to do, because I've never heard of or seen an aluminum A1 lower just break at that spot. The A2 style lower receiver is one of the details most frequently missed by novices putting together an A1 clone build.
