The US service rifle in use since 1965. The M16's original design proved to be jam prone (complicated by poor ammunition and improper cleaning instruction) and was superceded by the M16A1. The M16A1 was a fully automatic rifle with a chrome lined barrel for increased reliability.
The M16A2 was the next major model which had an improved sight, the optional addition of a brass deflector for left-handed shooters a 7-in-1 rifiling, and, most notibly, a three-round burst limiter.
The next model of the M16 (the M16A3 or M16A4 I cannot find reliable sources on either even Colt) eschews the burst limiter and adds a P-rail inplace of the handgrip.

Considered the largest leap in technological advancement of weaponry. The M-16 introduced the 5.56x45 which replace the large 7.62x51 used in the M14. The smaller round fired at a velocity of 3,100 feet per second and when entering the human body, it would fragment into 3 pieces creating an extremely large wound channel with a much lighter kick and more accuracy. M-16 was noted as an unreliable rifle during Vietnam. The reason for this is because of the ball powder that they used in the ammunition. It would swell up causing the casings to bend and stove-pipe in the chamber. After introducing the new stick powder ammunition, reliability problems faded away slowly. The M16 evolved from a light select-fire (semi-auto/auto) to the M16A2 which included a steel barrel, more rifle twist for increased accuracy and velocity, and three-round burst instead of automatic which although sounds worse, troops prefer three-round burst over automatic any day. The most recent addition to the armory was the M16A3(Auto)/A4(Three-Round Burst) which includes a removable carrying handle with a rail system on top of the reciever to mount any optic that accepts standard rails. The M16A4 also includes the KAC M5 rail system which allows you to mount lasers, flash lights, vertical grips, etc., allowing the M16 to be more of a modular weapon for each soldier.

The M16 is the standard issue assault rifle for the US military.

Manufactured by Colt, the M16 series fire the 5.56x45mm NATO round, which is accurate but lacks stopping power. It comes in safety, single, and 3-round burst trigger groups (except for the M16A3 which is automatic.) Auto fire was not used as it was deemed a waste of ammunition (and it usually is).

The original M16 was notoriously unreliable, due to low-quality ammunition, nagging issues with the ammunition feed, and the fact that the weapon was advertised as 'maintenence-free' (lol). The M16A1 introduced the foward assist, which was used to clear the front section of the gun in the event of a jam.

After Vietnam, new ammunition was developed, which required a modification to the rifling. This, along with other changes, created the M16A2.

Later, the M16A4 was adopted, featuring (among other things) full modding capability with Picatinny rails, which were already a feature on the M4A1.

Contary to popular belief, as long as you keep them clean the newer models of the M16 don't jam all that often. Also, unlike what someone else claimed, a good buttstroke with the '16 can ruin someone's day. It's quite a sturdy weapon.

The carbine version of the M16 is the M4 (burst)/M4A1(auto).

As defined in 'The guide book for Marines" 18 revised edition:

The M16A2 rifle is a 5.56X45mm, magazine-fed, gas-operated, air-cooled, shoulder-fired weapon. It is designed for either semiautomatic or three-round burst-control fire through the use of a selector lever. The barrel is surrounded by two aluminum lined fiberglass hand guards which are notched to permit air to circulate around the barrel and further serve to protect the gas tube. A super nylon butt plate is attached to the rear of the stock to partially reduce the effects of recoil. A forward assist assembly located on the right rear of the upper reciever permits manual locking of the bolt when this is not done by the force of the action spring. A spring-loaded rataining pin can be depressed with the nose of a cartridge to allow the trigger guard to be rotated down against the pistol grip to permit the firer ready access to the trigger when wearing gloves. A dust cover is provided to prevent dirt and sand from getting into the rifle through the ejection port. The dust cover should be kept closed at all times when the rifle is not being fired. It is opened automatically by the forward or rearward movment of the bolt carrier.

General data:
weight without magazine: 7.78 lbs
with 32 rd mag fully loaded: 8.79 lbs
length overall with flash compensator 21 inches.
standard ammunition includes ball, tracer and blank

The modernized, updated version of the AR15 platform, currently in use by the United States Army and Marine Corps. While in less numbers than the M16A2, and generally only used by frontline units, it is arguably better than its predecessor. The M16A4 features a picatinny rail, a railed fore grip, detachable sights, and a 1:7' rifling, rather than the 1:12' and 1:9' rifling previous models had, which equates to better power and accuracy. The M16A4 is, essentially, an M16A3, but with a safe/semi/3-round burst selector switch, rather than a safe/semi/fully-automatic switch.

Standard magazine capacity is 30-rounds of M855 or Mk. 262 5.56x45mm ball ammunition. It is gas-operated, with a rotating bolt. It is air-cooled, with an external, detachable magazine, peep hole sights, a forward assist, and a charging handle mounted on top of the receiver, behind the picatinny/carrying rail.

Reliability in the M16A4 is solid; one is capable of firing upwards of 1,000 standard, ball rounds through a decently maintained rifle, with STANAG magazines, without a single jam or malfunction. While disputably not as good as the AK platform, it is a far cry from the M16(A0), and is a step up from the M16A2.

The standard issue military assault rifle, fires 5.56 X 45mm round (.223) about 39 inches overall length, big brother to the M4A1. There are 4 different variations of the M16. M16A1(AKA VN), M16A2, M16A3, and M16A4(with removable carrying handle for optics)

"Poodle shooter" is a derogatory term for the M-16 and M-4 family of rifles with which the U.S. military is armed. This stems from the fact that the M16, M16A1, M16A2, M16A3 and M16A4 (the "M-4") fire the 5.56mm cartridge, a cartridge that many soldiers and Marines think is underpowered for combat operations. Prior to the Vietnam War, the Army and Marines were armed with he M-14 rifle, which fires a much more powerful cartridge, the 7.62mm NATO round. During the Iraq War and the war in Afghanistan, the M-16 and M-4 proved to be inadequate to the task of longer-range combat engagements and the M-14 rifle was brought out of mothballs and issued to some units.