The original M16 was a far cry from what you see today. The original M16 had a 1/12 or 1/14 twist, 20″ skinny “pencil” barrel, three prong flash suppressor, shot 55gr M193 ammo, no forward assist, triangle handguards, short fixed stock, and chrome plated and slick-sided bolt carrier. Selector switch, Safe, Semi, Auto.
The original M16 lower receiver had what is commonly referred to as a “slabside” receiver. It lacked the fence around the mag release that you see so often on AR15’s and M16’s these days.
The M16A1 was the same as the original M16, except for the addition of the forward assist, “fence” around the magazine release, chromed bore/chamber, and new birdcage flash hider (which was closed on the ends to prevent it from catching or snagging things), and 1/12 twist rate for better stabilization of the M193 cartridge in colder weather.
The chrome plated and slick-sided bolt carrier was replaced with a parkerized “notched” bolt carrier that worked with the forward assist. It was standard issue from about 1967 through 1985 or so.
Initial models usually featured 20 round mags. Some units today still get stuck with old M16A1’s, and this is the reason 62gr M855 has a green tip, to distinguish it between the 55gr M193 that is meant only for use in the older M16’s with 1/12 and 1/14 twists.
Introduced in 1983 and adopted by all branches a few years later, it featured a longer, stronger buttstock, round handguards, more complex rear sight with different sized apertures, brass deflector, a slightly different pistol grip, closed bottom birdcage flash hider to reduce dust signature and act as a compensator.
It also featured a heavier government profile barrel (light under the handguards) in 1-in-7 twist as a compromise of the best twist rate for the 62g M855 ammo (with a steel penetrator for better performance on Soviet helmets at 600 meters and light cover) and the M856 tracer round (which was much longer than the M855 round due it needing to store the tracer compound). Selector switch, safe, semi, and burst (3 round).
Most changes were prompted by the USMC. The switch to 3 round burst was added instead of full auto in order to promote more deliberate shooting and ammo conservation. The M16A3 would be exactly like this rifle only to include the option for full auto instead of 3-round burst. The M16A3 is used in some cases by the USN. The M16A2, although still in use was later replaced by the M16A4.
The M16A4 did not feature any radical changes as far as the operating system compared to the M16A2, but the two changes that were made have helped it to become a much better weapon system.
Changes include a flat top upper for mounting of optics or a detachable carry handle and KAC M5 RAS rail system. Like the M16A2, Selector switch is safe, semi, and burst (3 round). These rifles are being fielded mostly by the USMC and the vast majority of them are currently being made by FN (Fabrique Nationale) for the US Military. However, the full size M16 rifles like these are starting to lose favor in the military, as many units are getting more and more M4 Carbines.
XM177 – The First AR15 Carbines
Also known as the CAR-15 (Colt Automatic Rifle – 15) and the Colt Commando, these short barrel rifles were the first AR15 versions to see action in the US military in Vietnam.
Generally, they had a shorter (11.5″) barrel and collapsible stocks, and a different muzzle device called a moderator, although there were different models and variations. Early versions of this rifle lacked a forward assist while many later models featured one. They were very similar to the M16A1’s of the Vietnam era in other ways, such as selector switches (with some exceptions), barrel profiles, and A1 rear fixed sight, pistol grip, etc.
Other M16 Carbines were made as well, but these designs all eventually led to the development and use of the M4 Carbine.
The XM177 and M16 Carbines led to the eventual development of the M4 Carbine. Colt tried to determine the optimal shorter length carbine barrel and ultimately determined that (at the time) the shortest they could make the barrel without losing a considerable amount of reliability was 14.5″ long.
You may also notice that the M4 barrel has a distinctive “notch” cut in the front of it. This is in order to properly attach the M203 grenade launcher.
Most M4’s are also “light under the handguards”, meaning that the barrel is skinnier under the handguards, similar to how the current M16 barrels are.
Here you can see an M4 profile barrel. M4’s have adjustable buttstocks like the early M16 carbines, but they are an updated version. M4’s without rails have handguards that are “bigger” than most others and include dual heat shields.
It has many of the same features as the M16A4, including the flat top upper, A2 flash hider, pistol grip, and the same fire control group (Safe, Semi, Burst). M4A1’s are full auto instead of 3-round Burst and have a thicker barrel profile (under the handguards). M4’s are currently being made and issued by Colt (who has the contract with the US Military) and are seeing more and more use over the M16’s in the military, particularly in the US Army.
The M16 design has been altered and enhanced time and time again. Utilizing the inherent accuracy and the unmatched modularity, the M16 has been turned into an excellent sniper and counter-sniper weapon.
Squad Designated Marksmen (SDM’s) are becoming more and more common and they are often using slightly modified M16A4’s with scopes (often an ACOG), bipods, 2-Stage triggers and other things to enhance accuracy. They don’t act as snipers in the true sense of the word, but they do act as a more precise longer range rifleman.
The SPR (Special Purpose Rifle), also known as the Mk12 as named by the USN, is another M16 variant. Featuring an 18″ free floating match grade stainless steel heavy barrel, the SPR is usually fed with 5.56 Mk262 Mod 1 77gr OTM (Open Tip Match) Black Hills rounds. This load is known not only for it’s supreme accuracy but also for devastating fragmentation (when traveling at or above the fragmentation threshold of 2300fps).
A KAC (Knights Armament Company) 2-Stage trigger is used, as is a PRI free float tube and a Bi-pod. A typical scope is a 3.5–10x40mm Leupold.
There are some variations out there in the SPR family, including a variety of stocks, but you get the picture. Many AR15 enthusiasts make their own versions of the SPR based on the various features I mentioned. Similar to the SPR, the SR25, or Mk11 Mod 0, made by KAC is made for use by snipers. However, this rifle is chambered in 7.62×51 NATO.