Here's a refresher about what the various AR uppers look like and how to tell them apart, in case anyone wants to know.
1. The original AR-15 and M16 didn't have a forward assist, or jam enhancer as some people call it. Many people refer to these as slickside uppers.
2. The M16A1 did have a forward assist, but was otherwise the same as the older uppers.
3. The C7 had all the features of the M16A1, plus an M16A2 style built-in brass deflector. You can think of it as an M16A1.5. More than an A1, but less than an A2. The C7A1 is a flattop that has a Weaver rail that doesn't meet the M1913 Picatinny standard of the M16A4, but C7A2 flattop upper receivers are machined to the M1913 standard.
4. The M16A2 had the fully adjustable rear sight. The M16A3 used the same upper as the M16A2, but it had full-auto capability like an M16A1, instead of a 3-round burst like the M16A2. The M16A2 Enhanced rifle also has the same upper but it uses a 4-way Safe/Semi/Burst/Auto trigger group.
The M4 is a shorter and lighter variant of the M16A2 rifle and the M4A1 has a removable carry handle like an M16A4.
5. The M16A4 is the modern flattop upper with an M1913 Picatinny rail that works so well for mounting optics. It's hard to believe the most modern rifle we have uses a 111 year old rail from before World War I.
AFAIK the Colt M5 carbine was never adopted by anyone, but the front of the upper is octagonally shaped to match the free-floating M-LOK slotted handguard. It uses a short-stroke gas-piston instead of direct impingement. The Colt CMK rifle chambered in 7.62x39mm uses the same octagonal upper and handguard.
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