Things That People Ignore

The M16, M16A1, M16A2, M16A3 and M16A4 have 20" barrels.

Colt made several carbines based on the M16 series with 10-14.5" barrels for military service. So far, only the M4 has gotten wide issue.

Where did those carbine barrel lengths come from? The 10" barrel comes from the old Colt R607, the first carbine they offered. This is basically placing the gas port where the pressure curve has dropped to the point where it's just barely safe to open the bolt. Since the front sight base is common with the M16 and they left room for threads for a flash-hider it dictates the 10" length.

The 10" barrel was not very reliable because the pressure dropped very quickly once the bullet had passed the gas port. Colt introduced a 3.5" blast moderator to help with both the flash and to increase back-pressure for more reliable function. While it helped with the blast, further flash reduction and reliability was desired, so a 4.5" slotted moderator was introduced. This was carried over into both the XM177 and XM177E1.

Reliability was improved, but more "dwell", the time between the bullet passing the gas port and exiting the barrel, was needed. The M16 had 4.5" more dwell than any of the early carbines. Increasing the barrel length to 11.5" and using the 4.5" moderator made the XM177E2 even more reliable. Unfortunately, the Vietnam war ended and the funding for the project mostly dried up. Some issues were never resolved entirely.

Where did 14.5" come from? If you took the barrel that extends from the front of the front sight base on an M16 and put it in front of a front sight on a carbine, you have a 14.5" barrel. It is reasonably reliable and can accept an M16's bayonet; which the XM177E2 cannot. It also ends up being about 5/8" shorter than an 11.5" barrel with a 4.5" moderator! The SEALs adopted this 14.5" carbine; Colt called it the R653. The Philippines even manufactured them.

All of these guns used the same M193/M196 5.56x45mm ammunition that the M16 and M16A1 used. It was noted as far back as Vietnam that the shortened barrels had a dramatic effect on the effective range. Since the R607, and XM177 series were issued primarily to special warfare troops where the short length and lighter weight combined with different tactics from the infantry compensated for the lack of range.

The M16A2, M16A3, M16A4, M4 and M4A1 use M855/M856 ammunition.

Like the M193, the M855 was designed with a 20" barrel in mind. Like the R607, XM177 series and R653, performance dropped because of the reduced muzzle velocity from losing 5.5" of barrel. The shorter weapon was definitely handier for riding around in the back of a Bradley and the lack of long range was not a factor in urban environments.

Like so many times in the past, we ignored that the war we just fought, or the conditions we are currently encountering, will not be the environment of the next war or even engagement; thus the M4 became the primary issue service "rifle" for the Army. The Marines retained the M16A2 and adopted the M16A4. The current conditions in Afghanistan are screaming for something hard hitting with long range and even the Marines are having some issues reaching out to the ranges required. The M14 in 7.62x51mm has made a triumphant comeback in these conditions.

Will we abandon the M16 platform altogether for a 7.62x51mm rifle? Will we change the caliber of the M16 and M4 to something with a bit more reach? Will we develop something completely new? Will we look back at the rifle developed for the conditions in Afghanistan right now and wish we still had the M4?

EDIT
By they way: the 16" barrel we see on civilian versions of the carbines comes from a legal requirement.