Designed Right: The Colt CMG-3

Nathaniel F
by Nathaniel F

At this point, I’ve written relatively few articles about small arms design in general, due to my time going to a certain other writing project. However, small arms design and theory has always captivated me, since the beginning of my interest in firearms. Ian McCollum’s recent Forgotten Weapons video about the CMG-3 has really excited my interest, as the Colt machine gun design is a virtual incarnation of “best design practices”. The video, which includes disassembly and shooting segments, is embedded below:

So what makes the CMG-3 special, mechanically speaking? For optimum reliability, a small arm must be designed as a system; a common error in small arms design and engineering is “cargo-cult” design that seeks to incorporate as many features or desirable characteristics as possible without regard for how they are integrated into the system and how they affect the reliability and other characteristics of the system. Obviously, a holistic approach to small arms design is better, but having said that there are features that if properly integrated do substantially improve the reliability of a design, and the CMG-3 includes virtually all of the major uncommon ones. These are:

  1. Underlug. When a firearm unlocks is a critical aspect of how well it functions. Typically, the later a firearm unlocks, the less difficulty it will have in unlocking, due to the reduced pressure in the barrel. To facilitate as late an unlocking action as possible, a firearm’s mechanism may incorporate what is called “underlug” (a word which also has some alternate, unrelated meanings), which is a distance the unlocking device (usually a bolt carrier) is allowed to travel without acting upon – that is, unlocking – the locking piece (usually a bolt). Underlug is usually expressed as additional length cut into the front of the cam groove. The CMG-3 has what appears to be an inch or more of underlug, an exceptional – and possibly excessive – amount.
  2. Anti Pre-Engagement. In a locked-breech weapon, the locking piece is often actuated by the compression of the bulk mass of the operating group being compressed against the breechblock by the operating spring. Obviously, a weapon should lock when it goes into battery, and not lock when it’s not in battery, and so in for example a rotary bolt weapon, there must be something preventing the bolt from completing its turning move until it is engaged with the locking surfaces in the barrel or receiver. In an AR-15, the cam-pin is held in place by the receiver walls, but as the cartridge stack pushes on the bolt face during feeding this generates a certain amount of friction, that the action must then overcome. A weapon like the Kalashnikov or Garand, however, circumvents this by preventing the bolt from rotating when it is fully indexed in the feeding position. Both rifles feature a shaped lug which is pressed against a horizontal shelf cut into the bolt carrier, and so long as the bolt is in the vertical feeding position, no amount of force on it will cause it to try to rotate in such a way where it could rub or scrape against the receiver to cause friction. When the bolt reaches the position where it needs to rotate for locking, angles surfaces in the trunnion knock it out of position, and allow it to complete its turning move. While this is the most common incarnation of anti pre-engagement, the CMG-3 uses a different system. Its cam pin, similar to that on the AR-15, is spring loaded. When the bolt is fully extended in the feeding position, the cam pin is driven upwards by its spring, locking it into place in the bottom of the bolt carrier. As the bolt group moves forwards, the cam pin and a cross-pin mounted to it are driven upwards by surfaces in the receiver, allowing the bolt to rotate.
  3. “Broom”-Type Ejector. Removing the cartridge case from the action is critical for the functioning of an automatic firearm, but most firearms rely on momentum to actually eject a cartridge case from the action. This is a reliable and effective method of ejecting the case, but the CMG-3 and only a handful of other designs take it even further, and utilized a sweeping or “broom”-type ejector, that sweeps across the bolt face and physically pushes the cartridge case out of the ejection port. If a design can afford the additional mass and complexity of an extractor of this type, there’s certainly not a surer way to get spent cartridge cases out of the gun.
  4. Controlled-Round Feed. Mauser bolt-action rifles are famous for their massive claw extractors, which facilitate their reliable controlled-round feed action. In most firearms, and overwhelmingly in semiautomatic and fully automatic firearms, rounds are pushed by the bolt into the chamber, and in the final closing action a spring-loaded extractor snaps over the cartridge rim, ready to pull the case from the chamber. However, a controlled round feed firearm slips the cartridge case up a flat bolt face and under the extractor during the feeding cycle, allowing the bolt to control the round on its way to the chamber, for a very sure and trouble-free feeding stroke. By way of analogy, one can think of conventional push-feed as one American football player (representing the magazine) passing the ball (representing the cartridge) to another (representing the bolt). Controlled-round feed would then be that same player handing the ball off instead, a much more reliable way to transfer a ball between players. Controlled-round feed is not without its disadvantages; getting the round from the feed lips to the bolt face and under the extractor requires the round to be released from the magazine at just the right time during its travel forward; too early, and it may jump forward and not be captured by the extractor, too late, and it will jam between the bolt and the receiver. It’s for this reason that controlled-round feed is less commonly used in weapons with detachable, replaceable magazines, as feed lips with incorrect dimensions, either because of a manufacturing defect or damage, can cause severe malfunctions. It should further be reinforced that conventional push-feed is not “unreliable”, it’s simply uncontrolled. In the CMG-3, the extractor is not only designed for controlled-round feed, it is physically milled into the bolt face. On the one hand, this means the extractor is highly unlikely to break. On the other, it means that if and when it does, the whole bolt must be replaced.

Three years ago, Ian covered the CMG-2 machine gun, which is the 5.56mm parent weapon of the CMG-3. That video is embedded below:

Nathaniel F
Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at

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  • Noob Noob on Sep 22, 2015

    hmm. Does this broom type ejector mean that you could make a forward ejection system too?

  • Ben Ben on Sep 22, 2015

    Anyone else wondering how they expected extra drums to be carried? That is the most unwieldy drum I've ever seen.