If you really want the details, get Ezell's book, "The Black Rifle."
Skipping many of the details that I no longer remember, here's some of the highlights of the M16s variants:
M16 (SP1): Rear sight not height adjustable. No bolt-assist button on the right side of receiver. No fence around the mag release button. Chamber and bore not chrome plated initially. Barrel was 1:14 twist, IIRC. Safe, semi-auto, and full-auto on the selector.
M16A2: Rear sight adjustable for height and windage. Bolt assist on right side of receiver. Fence around the mag release button. Chamber and bore chrome plated. Barrel is 1:7 twist. Safe, semi, and 3 round burst on the selector.
M16A1: This came before the A2 and was something of a transitional model between the M16 and M16A2. I don't remember which of the features on the A2 were also on the A1. Check Ezell.
If you are buying a Bushmaster Carbine (not the dissipator), it is basically a post-ban, shorty M16A2. No bayo lug and a muzzle break, rather than a flash suppressor. I suspect the barrel twist is 1:9, but I'm sure Bushmaster's web site will have that info.
M1911
