Advanced Combat Rifle The Advanced Combat Rifle ( ACR ) was a United States Army program to find a replacement for the M16 assault rifle . The program ' s total cost was approximately US $ 300 million . Phase I of the program started in February 1986 when development contracts were placed with six companies : AAI Corporation , Ares Incorporated , Colt ' s Manufacturing Company , Heckler & Koch ( H & K ) , McDonnell Douglas Helicopter Systems ( MDHS ) , and Steyr Mannlicher . Phase I and II . 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 . Eugene Stoner ’ s Ares Incorporated also entered their Advanced Individual Weapon System ( AIWS ) , which used a 5mm tracer round , but had to withdraw due to ongoing problems . The AIWS shared some features with the Steyr entry , notably the " rising chamber " action and " telescoped " cartridge . MDHS . McDonnell Douglas Helicopter Systems , originally Hughes Helicopters , 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 fléchettes , first with five of them in a round , eventually three in a round . Phase III . Phase III began in August 1989 , when AAI , Colt , HK , and Steyr entered weapon testing . Although all the designs worked well , none managed to meet or even approach the 100 % improvement over the M16A2 that the program demanded . In 1986 / 7 , the United States Army Infantry School had published a report asserting that the rifle , as a weapon , had already reached its peak , and the only way to really improve matters was to use an exploding warhead . This led to the ending of the ACR program in April 1990 , and led the way to the Objective Individual Combat Weapon program . The ACR program was preceded by older programs such as the Special Purpose Individual Weapon . AAI . AAI Corporation entered the latest variant of their long line of experimental fléchette rifles . Their entry used a standard 5 . 56 x 45 mm cartridge case firing a 1 . 6 x 41 . 27 mm fléchette of 0 . 66 grams at 1402 m / s . One of the biggest complaints about their earlier efforts was the loud muzzle blast , a problem that is hard to avoid with a sabotted round . As a result , AAI added a flash hider / sound suppressor that reduced the muzzle blast to just louder than an M16A2 . While the standard 5 . 56 x 45 mm case was used , the rifle was not safe to fire using standard ammunition due to the design of the gas system . A special magazine was used to prevent soldiers loading standard 5 . 56 mm NATO ammunition into the magazine , but rounds could still be chambered by hand . The weapon was limited to three - round bursts despite the fact that one of the main reasons for using a fléchette is its low recoil . This particular design was less complex than some of their earlier models , which could switch between fléchette ammunition for rapid fire and standard 5 . 56 NATO rounds for long - range semi - automatic fire . H & K . The Heckler & Koch G11 series used caseless ammunition where the propellant was molded onto the bullet itself , making the round smaller and much lighter . The new K2 version used in the ACR tests held 45 rounds in a single long magazine lying along the top of the barrel , leading to a distinctive and somewhat blocky appearance . Steyr . The last entry submitted was the Steyr ACR , another fléchette - 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 bolt . When fired , the chamber would move down where a new round would be pushed in from the rear , forcing the spent cartridge case forward out of the chamber where it would drop out through an ejection port behind the pistol grip . The chamber would then move back into firing position on a spring , where it would lock in front of a fixed breechblock . On firing , the sabot traveled down the barrel with the fléchette and was quickly " stripped " off upon exit . 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 limited to three round bursts . Colt . One of the more traditional of the ACR prototypes was the Colt ACR , which was a highly modified version of the existing M16A2 . Modifications were the addition of a new optical sighting system , a hydraulic 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 design change was the use of " duplex rounds " , a single cartridge with two smaller bullets in it . Olin Corporation produced three different rounds for testing , the first consisting of two tungsten projectiles in a long - necked case , the second used a standard - length case with two , tungsten projectiles , and the final entry was another standard - length case with two projectiles , one , the other . The latter was eventually selected for submission to the ACR trials . The basic idea of the duplex load is to increase the number of projectiles fired , which is the primary determinant of battlefield casualties . However , they significantly reduced accuracy , requiring the user to also carry traditional ammunition for long - range shots .