Since the introduction of the AR-15/M16 series weapons in the early 1960s, the weapon was envisioned by Colt to be a versatile family of weapons that could serve any purpose required for the mission. Initial designs were for a standard infantry rifle followed by the shorter and more compact carbines. However, there appeared to be a gap in the light machine gun area. Colt’s early experimental models began in the late 1960s with the M16 Rifle HBAR M1 and as of 2009 Colt Defense introduced the Infantry Automatic Rifle (IAR6940).
Early attempts at a longer sustained rate of fire began with the M16 Rifle HBAR M1 (R0606) and the M16A1 Rifle HBAR M1 (R0606A). Both were equipped with a heavy barrel and the only difference between these weapons is the R0606A has a forward bolt assist. Both were selective fire with semi and full automatic settings. Additionally, Colt built the R0606B which utilized the Foster Sturtevant designed (US Patent No. 3,292,492) four-way selector offering semi, fully automatic and 3-round burst modes of fire. All these weapons fired from the closed bolt position and utilized standard triangular handguards. These models were developed mainly for the SAW trials. Later models of early generation heavy barreled light machine guns included the R0613M16A1 HBAR with an untapered barrel but still utilized the triangular hand guards.
During the same time period, there were several experimental belt fed variations of the rifle. These would include the Colt belt-fed M2 HBAR and the CAR-15 Heavy Assault Rifle M2. Small numbers were made, primarily to be evaluated as helicopter door armament (November 1964).
The first dedicated LMG began in the mid 1970s as part of the SAW (Squad Automatic Weapon) program. This was called the XM106 and had two main departures from what Colt had previously worked on. First, this utilized a removable barrel using a lever that was pushed downward and the barrel assembly would slide out the front of the upper receiver. The gas tube was attached to the barrel assembly along with a small triangular handguard assembly to prevent the shooter’s hand from being burned when the barrel was replaced. The upper receiver of the weapon had a vertical pistol grip and a folding bipod. The second, and most significant modification of the weapon, was this one fired from the open bolt position. The hammer was the sear to hold the bolt open until the trigger was pulled.
The First Production Colt Open Bolt Machine Gun
Rob Roy, then in charge of military sales, initiated the development of an additional member of the growing M16 family of weapons to include a light machine gun that would maximize as many parts of the current weapon system as possible. The early stages of development of this new weapon were prior to the design and release of the new M16A2 rifle, but some of the developments of the LMG would surely impact the design of the M16A2.
The LMG would fire from the open bolt and fire only fully automatic. There are benefits to the open bolt mechanism; with the biggest advantage being to prevent cook-offs as there is no round sitting in the chamber. This also aids in cooling of the weapon by allowing air to circulate in and out of the bore. The task of developing the open bolt mechanism was given to one of Colt’s finest designers, Henry (Hank) Tatro. On February 28, 1984, Tatro was granted US Patent No. 4,433,610 for his “Open Bolt Mechanism for Automatic Firearm.” This was a truly unique mechanism. The standard M16 lower receiver is used and the only difference is the marking on the fire control: only Safe and Fire positions, no semi option. However, the fire control components could be dropped in any selective fire lower receiver. One of the most fascinating features of his design is the hammer serves two purposes. First, it is the sear which engages the bottom of the bolt carrier to hold the bolt in the open position and the second function is the hammer also strikes the firing pin to fire the cartridge. The Tatro-designed fire control group consists of a hammer, trigger, connector, selector, automatic sear and bolt carrier. The selector only has two positions. The automatic sear serves two functions: pulls downward on the hammer to release the bolt carrier group and also releases the hammer when struck by the bolt carrier to fire the weapon. The connector is the link from the trigger to the automatic sear used to pull the hammer downward to release the bolt carrier.
The Colt LMG Open-Bolt Cycle of Operation
The charging handle is retracted, pulling the bolt carrier group all the way to the rear where the hammer sear will engage the notch on the bottom of the bolt carrier holding the bolt carrier group in the open position. A loaded magazine is inserted into the weapon. With the safety in the Fire position the trigger is pulled. When the trigger is pulled, the pin on the trigger lifts the connector, which in turn rotates the automatic sear, which lowers the hammer out of engagement with the bolt carrier releasing the bolt carrier group forward. With the forward movement of the bolt carrier group the bolt lugs strip a cartridge from the magazine and feed it into the chamber. As the bolt moves into the locked position the sear trip on the bottom rear of the bolt carrier tips the automatic sear releasing the hammer to strike the firing pin.
The firing pin strikes the primer firing the cartridge. As pressure increases the projectile moves down the bore. Once the projectile reaches the gas port located under the front sight assembly, a portion of the gas flows up the port and into the gas tube. The gas then enters the bolt carrier key and then into the expansion chamber located in-between the back of the bolt and bolt carrier. This evenly distributes the gas pressure creating a hammer like blow pushing the bolt carrier group rearward and simultaneously unlocks the bolt. As the bolt carrier group moves rearward, the fired cartridge case is extracted from the chamber and ejected out the ejection port. As the bolt carrier group continues to move rearward, the hammer is pushed down and engages the automatic sear where it is held. When the bolt carrier group reaches full rearward travel, the recoil spring thrusts the bolt carrier group forward stripping another cartridge from the magazine and the cycle repeats. If the trigger is released, the connecter rotates again allowing the hammer to rise and catches the bolt carrier and holds it to the rear. If the magazine runs out of ammunition, the bolt catch will lock the bolt carrier to the rear. When the magazine release button is pressed, the magazine catch pushes outward on the extended arm of the bolt catch releasing the bolt carrier group to the hammer. The weapon is now ready to fire once the magazine has been changed.