Colt M16 (USA)

Dissatisfaction in the "Battle Rifle" concept of US Army in the late 1950's caused an issue for lighter selective-rifle with smaller caliber. One of the such rifles offered for the US Military was AR-15, designed by Eugene Stoner at Armalite. This rifle was in fact scaled-down AR10, redesigned to fire new small-caliber cartridges .223 Remington (developed at Sierra Bullets; based on .222 Remington hunting cartridge). AR15 have same gas-driven rotary-bolt mechanism without the gas piston, as an AR10. Armalite sold this design to Colt, and when US Army adopted AR15 in 1957 under official designation of M16, all further modifications were made by Colt. Then-new M16s were sent to Vietnam, where many problems were found. Due to "dirty" design (powder gases vented directly into receiver) M16 were very prone to stoppages and jams when insufficient cleaning was applied. Also polymer hand-guards, grips and butt-stocks were found too fragile for battle environment. In 1967 M16 was modified into M16A1 version. Contrary to popular belief the forward assist was always on the Army/Marine M16s, which was used to close the bolt in case of cartridge feed failure (so called "forward assist device"). 

In early 1980s M16 was modernized again, now into the M16A2. Major upgrade was new, heavier barrel with new rifling, to accept new cartridge SS109 with heavier bullet (designed in Belgium by FN). This upgrade gave the bullet fired from M16A2 more flat and stable trajectory. Other upgrade was done to fire selector - new mode of fire - 3 round burst was added to single-shot.  The full-auto mode was removed, in my opinion that was a big mistake. New dual-aperture peep-hole sight with both elevation and wind-age adjustment was also installed, along with new circular-cross-shaped hand-guard instead of the old triangular-cross-shaped one.

Latest modification of AR15/M16 family is M16A4, which is exactly the same as M16A2 except for integral carrying handle, which is replaced by Picatinny-Weaver rail system to accept different types of scope mounts, laser-aiming devices etc. Carrying handle with iron sights, similar to that of M16A2 also could be installed. The Marine Corp recently announced that they will replace their M16A2s with M16A4s. Contrary to the Army which seems to be moving towards replacing their M16A2s with the M4.

While being popular military rifle (adopted by nearly 30 countries, including USA, Canada, Israel and others), "civilian" and "law enforcement" clones of the M16 also gained significant popularity. Being marketed under many designations (such as AR15, M15, XM15 etc.), those clones are manufactured by many US and other countries companies, such as Bushmaster, Armalite, Professional Ordnance and others. Those guns may reproduce original M16s in everything excluding semi-auto trigger mechanism, or represent some improvements or tuning options (barrel lengths and styles (light, heavy, match)), butt stock types etc.

Colt M4 (USA)

Caliber: 5.56 NATO (.223 rem)
Action: Gas operated, rotating bolt
Overall length: 838mm
Barrel length: 368mm
Weight: 3.405kg
Muzzle velocity: 907m/s
Muzzle energy: 1684J

M4/M4A1 - The M4 is a lightweight, gas operated, air cooled, magazine fed, select-fire (like the A2, 3shot burst and semi), shoulder fired weapon with a collapsible stock. A shortened variant of the M16A2 rifle, the M4 provides the individual soldier operating in close quarters the capability to engage targets at extended range with accurate, lethal fire. The M4 Carbine achieves over 80% commonality with the M16A2 Rifle and will replace all M3 .45 cal. submachine guns and selected M9 pistols and M16 series rifle. Newer production versions no longer have an integral carrying handle, it is replaced by Picatinny-Weaver rail system for mounting different sighting devices (as on M16A4). M4A1 is exactly the same M4 except that is a true full-auto weapon, without a 3shot burst, like the original M16s. This is a specific request of the Special Operations Command.  After all any trained professional can fire three shot burst with a standard M16 and sometimes it is necessary to lay down a wall of fire...

Alliant Techsystems / Heckler & Koch OICW/XM29 (USA)

Caliber: 5.56 mm NATO (KE) and 20x85mm (HE)
Action: Gas operated, rotating bolt (KE), unknown (HE)
Overall length: 890 mm
Barrel length: 250 mm (KE) 460 mm (HE)
Weight: ca 5.5 kg empty; ca 6.8 kg loaded
Magazine capacity: 30 rds box (KE) and 6 rds box (HE)

The OICW (Objective Individual Combat Weapon) is very ambitious project of the US military that should enter the service circa 2009. Initially, two teams were selected to made prototypes, but in the 2000 one team, leaded by Alliant Techsystems Inc., won the contract for further development. This team also consist of Heckler&Koch, Dynamit Nobel and some other companies.

The key idea behind the OICW program is to increase effectiveness of the soldier and the kill probability. To achieve this, the OICW has three major components: the Kinetic Energy (KE) module, which is, in fact, modified Heckler-Koch G36 assault rifle, High Explosive (HE) module, which is a semi-automatic, magazine fed 20mm grenade launcher, and computerized day/night sighting system with integrated laser rangefinder and fire control unit. The system is anticipated to be 5 times more lethal than current M16A2 / M203 combo.

The design of the KE module has nothing new behind it. The only one unusual thing is a set of buttons on the left side of the pistol handle. These buttons control fire computer and laser rangefinder, as well as selection of fire mode (KE or HE).

The HE module fires 20mm high-velocity grenades. These grenades had programmable fuses, that could be set for Air Burst mode (grenade explodes at the specified range above the target; range is set via Fire Control module from laser rangefinder), or for Point Detonation (grenade explodes upon impact).

For obvious reasons the KE module could be separated from HE / Fire Control modules and used as a typical assault rifle with iron sights, but without butt stock. The separated HE module is completely useless since it uses KE module trigger to operate.

This weapon is still in the developmental stages so the look and feel, as well as the requirements will change considerably until it is fielded by U.S. troops.

Alliant Tech Systems (ATK), systems integrator on the OICW/XM29 project has announced in a press release the awarding of a $5 million contract by the U.S. Army Armament Research, Development, and Engineering Center (ARDEC), Picatinny, N.J., for a "rapid development program" called the XM8.  The U.S. military is investigating the lower part of the OICW (essentially a G36K) as an M4 comparable stand alone assault rifle for selected troop deployment, for those fielding the XM29/OICW (not general troop M16 replacement).  

Armalite AR-18 (USA)

Caliber: 5.56x45 mm NATO (M193)
Action: Gas operated, rotating bolt
Overall length: 940 mm (738 mm with folded stock)
Barrel length: 464 mm
Weight: 3.09 kg with empty 20 rds magazine, 3.317 rg with loaded 20 rds magazine
Magazine capacity: 20, 30 or 40 rds

AR-18 was designed by Armalite company, USA, in 1963, as a competitor to M16. The AR-18 had stamped "folding" receiver, and gas drive with gas piston, which is more reliable than direct gas system of M16, especially in adverse conditions (proven many times by AK-47). The lack of the recoil damper enabled implementation of the side-folding butt stock. The charging handle is located on the bolt carrier, at the right side of the weapon. AR-18 was select-fire weapon, while AR-180 was semi-auto "civilian" variation.

Initially manufactured in USA by Armalite, later license to this design was sold to HOWA company of Japan and to Sterling company, Great Britain. Total of no more than 20.000 AR-18s and AR-180s were made.

Many refer the AR-18 as excellent gun, and it's failure had the roots in the political situation - at that time (1960s-1970s) USA and USSR gave away millions of M16s and AK-47 to everyone who claimed to be "friendly" to one or another country, so there were almost no place for AR-18. As of note, AR-18 was quite popular among terrorists from IRA, who gave to the rifle the name "Widow maker".
Armalite has released a re-incarnate of the AR-180. It is called AR-180B, and will be similar to original AR-180. The biggest difference is the new polymer lower receiver with magazine well and trigger group that is similar to those from Armalite's M15 rifle (their own variant of AR15). This will give to the new rifle partial parts interchangeability with other AR-15 style rifles, as well as ability to use ALL M16/AR15 type magazines.

Though never adopted officially by the U.S. Military, Many of today's modern assault rifles are based on this action. Such as the SA80, FNC, and several others.