The Advanced Combat Rifle was a US Army program to find a replacement for the venerable M16 assult rifle. Phase I of the program started in February 1986 when development contracts were placed with six companies, four of which eventually entered testing in August 1989 during Phase III. Although all the designs worked well, none managed to meet the 100% improvement over the M16A2 that the program demanded. In 1986/7 the US Infantry School published a report claiming that the rifle had already reached its peak, and the only way to really improve matters was to use an exploding warhead. This lead to the ending of the ACR program in April 1990, and the starting of the Objective Individual Combat Weapon program. The Army is the branch of the United States armed forces which has primary responsibility for land-based military operations. ... M16 can mean: M-16 (rifle) Eagle Nebula This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... M16A2 assault rifle. ... The Objective Individual Combat Weapon or OICW usually meant the next-generation assault rifle competition that was under development as part of the OICW program. ...

Simplest of the ACR prototypes was the Colt ACR, which was essentially a modified version of the existing M16A2. The main external modifications were the addition of a new optical sighting system, a buffer to smooth out recoil during automatic fire, and a collapsing butt stock similar to the one already in use on the carbine versions of the M16. The key internal improvement was the use of "duplex rounds", a single cartridge with two smaller bullets in it. Olin produced three different rounds for testing, the first consisting of two tungsten projectiles in a long-necked round, the second a standard-length case with two 27 grain .158" tungsten projectiles, and the final entry was another long-necked round with two .224" rounds, one 35 grains the other 33. The basic idea of the duplex load is to increase the rate of fire, which is the primary determinant of battlefield casualties. However they also significantly reduce accuracy, requiring the user to pack two types of ammunition for long-range shots. Since the Colt ACR used non-standard length cartridges, this would have meant having to produce new "singlex" rounds as well. General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ...

One of the most distinctive, and well known, of the ACR entries was the Heckler & Koch G11, in a new version known as the G11 K2. The G11 series used a caseless ammunition where the gunpowder was molded onto the bullet itself, making the round smaller and much lighter. Unfortunately it was later realized that the brass casing normally used carries away a considerable amount of the heat of firing, and without the casing the early model G11's tended to overheat and eventually "cook off" their ammunition. By the time of the ACR contest these problems had been fixed. The new K2 version held 45 rounds in a single long clip lying along the top of the barrel, leading to a distinctive and somewhat blocky appearance. HK G11 assault rifle, one of the earlier models. ...

AAI entered the latest variant of their long line of so-far unsuccessful flechette rifles. Their entry used a 5.56 x 45 mm round weighing 9.1 grams firing a 1.6 x 41.27 mm flechette of 0.66 grams at 1402 m/s. One of the biggest complaints about their earlier efforts was the loud sound, a problem that is hard to avoid with a sabotted round, so for this version they added a well developed flash hider/noise supressor that made it just louder than an M16A2. A new magazine was used to avoid soldiers loading standard ammunition into the clips, but rounds could still be chambered by hand and this presented a potential problem. Oddly the weapon was limited to three-round bursts, oddly because one of the main reasons for using a flechette is that its light weight makes very high cyclic speeds possible due to low recoil. This particular design was less advanced than some of their earlier models, which could switch between flechette ammunition for rapid fire and standard 5.56 NATO rounds for long-range semi-automatic fire. The word flechette is French and means dart (literally, little arrow). It is a projectile having the form of a small metal dart, usually steel, with a sharp-pointed tip and a tail with several vanes to stabilize it during flight. ...

The final entry was the Steyr ACR, another flechette-firing weapon. The Steyr differed from the AAI in the details of the round, which used a plastic shell casing to reduce weight. The firing mechanism was quite complex as a result, moving the entire chamber as opposed to just the block. When fired, the chamber would move down where a new round is pushed in from the rear and forced the old round out of a hole in the front of the rifle. The chamber would then move back into firing position on a spring, where it would lock in front of a fixed block. On firing the sabots travelled down the barrel with the flechette and were quickly "stripped" off on exiting. This was found to present a hazard in combat, where the sabots could hit other soldiers or bounce off the ground when being fired prone. Like the AAI weapon, the Steyr was also limited to three round bursts. The Steyr ACR (Advanced Combat Rifle) has a superficial resemblance to the Steyr AUG, but instead fires 5. ...

Two weapons were cut from the list before Phase II started. The companies started an appeals process and were eventually re-instated, but too late to see testing before the ACR program ended.

McDonnell Douglas Helicopter, originally Hughes Aircraft, planned to enter their design using a plastic-cased cartridge they called a chiclet due to its box-like profile. Their first loads used duplex or triplex loads of normal projectiles, but the recoil was too high so these were replaced with flechettes, first with five of them in a .42 round, eventually three in a .338 round.

Eugene Stoner's ARES also entered their Advanced Individual Weapon System (AIWS), which used a 5mm tracer round, but had to withdraw due to ongoing problems. The AIWS shares some features with the Steyr entry, notably the "rising block" action and "telescoped" cartridge. It has been suggested that this article or section be merged with M16_(rifle)#Eugene_Stoner. ...