When I enlisted in the USMC I was issued a M16A2. Other then when we occasionally added a PEQ-2 IR laser, a flashlight, or the issued M203 grenade launcher, we felt this gun did everything we needed it to do.
We didn’t know about things like quad rails and quick detach accessories. Those introduced a whole different mentality to how to use our weapons.
When the M16/M16A1 become the M16A2 it gained about a pound. From the M16A2 to the M16A4 it gained about another pound. Most of this comes from the Knights Armament Corp. (KAC) M5 RAS rail system. The KAC M5 RAS is a good rail system, but it has been surpassed by much lighter better free float rails.
That additional weight, combined with the weight of various lights, lasers, and optics along with the fixed stock not playing well with our body armor made using the M16A4 a great deal more awkward. There was a huge difference between just wearing ALICE gear and shooting a M16A2 verses wearing an Interceptor vest with plates and MOLLE II gear shooting a M16A4 with ACOG, PEQ-2, and a Surefire.
Since I got out of the military, I have owned several rifles similar to the M16A4. I had a Bushmaster, a BCM, Saber, and now a Colt AR15A4. Most of these guns I shot for a while then got rid of. The reason for that is that the M16A4 is not particularly special at anything.
I have often told people that the M4 is a jack of all trade, but master of none. Truthfully, the M4 really excels at many of the roles it is used in. The M16A4 type rifle falls into an odd place where it doesn’t particularly do any one thing significantly better than the M4, yet is inferior in handling and weight.
The M16A4 is not a precision rifle. While it is more than accurate enough for combat, it is not a sniper rifle. If your planning to shoot in Rifle Competition, you would be better off with a rifle with a fixed carry handle with match sights, and a free floating match barrel.
While the M16A4 isn’t overly large or heavy, it certainly is not as handy for shooting indoors or confined spaces. The M4 and smaller guns show distinct superiority in handling while in close quarters shooting conditions.
While I was in the Corps we were generally of the belief that the M16A2/A4 was a superior weapon system to the M4. Statements of increased accuracy, reliability, and lethality (due to increased muzzle velocity) were occasionally thrown around.
When using Iron sights, the longer sight radius of the M16 is clearly apparent over the carbine. However most of us no longer use iron sights as our primary way to aim, so this is a moot point. As for mechanical accuracy, the M4 doesn’t give anything up to the M16.
Around 2005-2008 or so I often recall reading on the major gun forums that the AR15 rifle is so very much more reliable than the Carbine. We do know that the rifle has lower gas port pressure and should be easier on components than the carbine. Still for most users they will not see a reliability benefit from the rifle. Not to mention the carbine is clearly reliable enough for groups like the SEALs, U.S. Army, and various foreign special forces groups. So back in 2006, the M4 was good enough for the US SEALs, but not a good enough weapon for the USMC infantry.
While I was in there was a push to give the Marine Infantry M4 carbines. This was considered foolish. Later after I was out there was a push to make a “Product Improved” M16A4 with features like a free floating rail, and a compressible stock. I have heard that from several sources that the USMC decided instead to just give infantry Marines M4 carbines. In 2007, shortly before I got out, one of the other unit got brand new M4 carbines. When I inquired why, I was told that one of John McCain’s son was in that unit. I don’t know if that is true, but it sure seems like the type of reason one unit would get superior equipment. Clearly the M4 is good enough for the USMC now.
I like the M16A4 configuration, but I own one more for plinking and nostalgia. I firmly believe that the majority of AR15 user would be better off with a good carbine than the M16A4 configuration.