The original design for the M16 had a number of flaws: lack of cleaning kit, no chrome lining on the inside of the barrel or chamber, issues with cartridges etc. These flaws became more evident during the Vietnam war, but sadly, only after a number of US servicemen lost their lives. Soon after a Congressional investigation, fixes were made for all these issues and the rifle became a much more reliable product. However, the bad reputation earned by its early flaws took a lot longer to clear.
By 1970 though, the M16's reputation was established and other countries also wanted their own M16 style rifles. The NATO member countries decided to standardize on the 5.56x45 mm. cartridge, which was also used by the M16 A1 model, but they wanted to make improvements to the US version of the cartridge, in particular, because of improvements to body armor technology. The US version of the 5.56x45 mm. cartridge was called the M193 and tests had shown that it didn't penetrate newer body armors that well. The British, Germans and Belgians all presented cartridges that all had the same 5.56x45 mm. external dimensions, but had different bullet shapes and materials that offered much better ballistics and penetrating power than the M193. In the end, the Belgian designed SS109 (which has a bullet with a steel tip) was chosen as the NATO standard. The Belgians also designed a tracer round, L110, which had better performance than the old M196 tracer round. The M196 could burn out in 450 meters, whereas the L110 could show its trail to 800 meters. However, it was found that the M16 A1's barrel rifling twist rate of 1/12 inch (i.e. 1 turn every 12 inches or 1 turn every 300 mm.) was not adequate to stabilize the SS109 or L110 bullets past about 90 meters or so. New barrels with a faster twist rate were required to get the best performance out of the SS109. The ideal rate for a bullet from a SS109 cartridge is at least 1 turn in 9 inches (1 turn in 229 mm.), but the L110 requires at least 1 turn in 7 inches (1 turn in 180 mm.) to continue to be stable for much longer ranges (over 800 meters or so). Hence, it was decided to make a new version of the M16, the model A2, which would feature a barrel with a 1 turn in 7 inch twist rate.
It was also found during the Vietnam war, that many soldiers would put their M16s into full-automatic mode and when in a firefight, new soldiers would often hold down the trigger and shoot entire magazines into the bushes in a few seconds, without killing a single enemy. This caused many of them to run out of ammunition prematurely. Also, the act of firing in full-automatic mode makes the rifle more difficult to control and thereby reduces accuracy. The US military conducted various tests a decided to replace the full-automatic firing mode with a three-round burst mode (i.e. upto 3 rounds would fire when the trigger is held down), because the three-round burst was determined to be the optimum balance between accuracy, ammunition conservation and firepower. The US Marines were the first to request the development of the M16A2 model based on the Vietnam war experiences.
M16A2 model. Click on image to enlarge. Public domain image.
Along with the newer barrel with the 1:7 inch twist rate and the three-round burst mode, there were numerous other improvements made to the M16A2. The thickness of the barrel in front of the front sight was made thicker, to avoid bending the barrel in the field and to also resist overheating better, thereby allowing firing the weapon for longer periods of time. The front and rear sights were improved and the rear sight was made adjustable for ranges between 300-800 meters using the SS109 cartridge (which had different ballistics than the M193). The recoil compensator at the end of the barrel was closed at the bottom, so it would not kick up dust when fired in the prone position. A deflector was attached immediately behind the ejection port, so that left handed users would not get hit in the face by a hot ejected case. The grips were improved and the handguards were made symmetrical so that there was no need to manufacture spare parts for left and right side. With advances made in polymers, the buttstock was re-engineered to use a new plastic that is 10 times stronger than the original stock on the M16A1. The buttstock was also made slightly longer than the original version.
The M16A2 model was first adopted by the US Marine Corps in 1982, followed by the US Army in 1986 and then the other branches of the US military, as well as military forces of other countries worldwide. It is still being used by many users and is the most common variant in the M16 family.
The M16A3 model came out around the same time as the M16A2, but was made in very small numbers indeed. The M16A3 is very similar to the M16A2, but retains the original full-automatic firing mode of the M16A1 instead of the three-round burst mode of the M16A2. This variant was made for US Special Forces, who need the extra firepower afforded by the full-automatic firing mode. Special forces personnel are trained to maintain better fire discipline and not use up their ammunition so quickly. The US Navy was the first to order this variant, for use by its SEAL and Seabee units.
The M16A4 is the latest variant of the M16 family.
M16A4 rifles. Click on image to enlarge. Public domain image.
This is currently being issued to US Marine Corps front line units, and some US Army units. The main difference between this and the M16A2 is the removal of the fixed carrying handle/rear sight, which is seen on the M16A2. Instead, this is replaced by a Picatinny rail on the A4 model. This allows the user to not only attach a carrying handle, but also other accessories, such as a scope, as seen in the image above. The hand guards in front of the magazine are also of a new design (Knight Armament's M5 RAS handguard), which allows the user to attach a vertical grip, tactical light, laser sight etc.
The M16 rifle had initial teething issues, but has later evolved into a fine, reliable weapon, which is in use by several military forces around the world.