Operating Systems 101: Introduction and Foundation

What makes an automatic (or semiautomatic) weapon work? How do these weapons accomplish being able to fire round after round through a single barrel with no interference from the operator? That’s what I aim to explain in this series, which hopefully will give my readers a brief and readily accessible foundation on firearms operating mechanisms. For some of you, many of these articles will not tell you anything you don’t already know, but be patient: I plan to be very thorough in my coverage of different operating mechanisms. The scope of this series will be the operating mechanisms of single-barreled firearms, minus their feeding mechanisms (which I will cover separately).


Slow motion footage of a Pedersen rifle completing a full cycle of operation. Note the firing, extraction and ejection of the spent case, and feeding of the next round.


First, though, we need to explain what is happening generally-speaking when an automatic* firearm is operating. Therefore, we’ll need to define some terms:

  1. The barrel is the primary pressure-bearing element, which also directs the bullet downrange and, if rifled, spins the bullet to stabilize its flight. The uprange end of the barrel contains the chamber, where ignition of the cartridge takes place.
  2. The breech is the closed end of the barrel that contains the rearward pressure created by the cartridge. In almost all automatic firearms it is a movable piece, to allow reloading of the chamber.
  3. The moving parts group is the mechanism which uses motion to accomplish repeated fire from a single barrel, in conjunction with a feeding device. Some single-barrel multishot firearms do not use a moving parts group, but we’ll cover them later. The moving parts group often – but not always – also incorporates the breech.
  4. The feed mechanism is the device which presents fresh cartridges to the operating group. For the purposes of this series, we will be mostly ignoring how feeding is accomplished.
  5. The fire control group is the mechanism which is capable of repeatedly activating cartridges once they have been feed from the feed mechanism into the barrel by the moving parts group. Fire control groups will become a much bigger subject later on in the series.
  6. A cycle is one complete round of fire, from igniting a cartridge all the way to just before ignition of the next cartridge. This word cycle also gives us the term “cyclic rate of fire”, meaning the rapidity with which the firearm can complete repeated cycles.

We’ll cover more definitions as the series carries on. Stay tuned for the first episode, on the blowback operating mechanism!

*Note: For the purposes of this series, we will wrap up fully automatic and semiautomatic firearms together under the label “automatic”, unless distinction is needed.

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. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


  • Simon

    I’m excited for this

  • Isaac Newton

    The definition for “moving parts group” may make it difficult to explain the how something like an inertia operated Benelli works. Its almost too broad.

    For the Benelli, operation requires everything other than the carrier to “move”. It may sound like this: “the barrel, breach, fire control group, and stock move back while some parts of the moving parts group remain stationary.”

    • roguetechie

      He actually left a provision open for this in saying some weapons have non moving parts that typically move in others. One thing is certain, things like inertia operation and delayed blowback are very advanced concepts. Their apparent mechanical simplicity only comes at the price of very intensive and rigorous development. My grandpa always told me with enough parts you can make anything work, but truly gifted minds eliminate parts to make masterpieces.

      He was an incredibly intelligent and humble guy who taught me so much more than mechanical stuff. His feelings on simplicity were echoed in every area of his life, whether you want to build a good gun or a good life simplicity seems to be a great starting point.

      • DB

        Parts left out cost less and cause no service problems. In other words, simple is generally better

      • Longhaired Redneck

        “(W)ith enough parts you can make anything work, but truly gifted minds eliminate parts to make masterpieces.”

        Your grandfather is a wise and observant man. When I read your post I thought immediately of John Moses Browning. With your permission I will pass the quote along.

        • roguetechie

          Please do pass it on! I can’t think of a better way to honor my grandfather than to keep his words alive. Truly a good man who didn’t talk much, but could have even teenage me hanging on every word because of the wisdom and thoughtfulness behind them.

          It makes me happier than you would imagine that you took time to write a response.

  • PK

    Excuse me while I point each and every single firearms novice asking questions to this page…

    This is going to be a wonderful primer to suggest as starting material, I can tell.

  • TechnoTriticale

    re: we will wrap up fully automatic and semiautomatic firearms together under the label “automatic”

    The more traditional phrase “self-loading” would cover both. “Automatic” is ambiguous; hence your clarifier.

  • Sasquatch

    Nice article.

  • Major Tom

    This sounds like it could be fun.

  • David

    I saw the picture for this article before I read the headline and thought, wait another article about a toggle rifle? Not that I mind toggles, they’re neat, but this is a nice change.

  • MPWS

    I found to my surprise, that thru years of my involvement with the subject, I have now more interest in manual operated repeaters – they are and should be the starting point. This is not to say that semi and full auto firearms are out of my field of view, but having more appreciation of former I am somehow closing the learning cycle.

  • politicsbyothermeans

    Definitely looking forward to this.

  • Anton Gray Basson

    Its like the start of every South African Firearms Competency manual

  • A very good video done by the military that show the various methods of operation this one.I had my nephew watch it so he could better understand the guns we were shooting and cleaning.

    • PK

      I love that video! All of the in-depth training videos from that era, while some downplay the dangers faced by troops (the one about not fearing the MG42 comes to mind), are all quite enjoyable.

  • Volk

    Is that Ian’s footage? Oh and we may still learn, even if it’s a small detail or bit of trivia.

    • Yes, that’s when Ian came out and shot the Pedersen with TFB.

  • Joe Goins

    A to the MAZING!

  • DIR911911 .

    press trigger , gun go boom . . . repeat

  • Here’s what we’ve got in the pipeline, folks; all of these have been written and scheduled:


    • ostiariusalpha

      After having Pete M. bring up the matter, I hope you can perhaps cover some of these, if possible:

      Operating Systems 201: Toggle Lock vs. Tilting Barrel

      Operating Systems 201: Rotating Barrel (Short Recoil)

      Operating Systems 201: Locking Block (Short Recoil)

      (Or you could just combine them as:
      Operating Systems 201: Locking Block and Rotating Barrel)

      Covering the rotating barrel, with the various methods applied in the 1897 Browning design (US580,925), Steyr-Hahn, Obregón, Beretta 8000/Px4, and Mauser M2, would be fascinating. The locking block system would be really insightful also, such as those found in the Feederle brothers’ Mauser C.96, the Feederle-Nickl pivot block (used in the Walther P.38, and the Beretta M1922, M1951, & the 92/M9), and the Bergmann system seen in the Lahti L-35/Husqvarna M/40, and the Arsenal Firearms Strike One. It would be perfect if you could maintain the pithy and general tone of these articles, without getting bogged down in minutiae of each individual firearm, except as short examples. Thanks for this series, I’m already enjoying it a lot!

      • Locking mechanisms will be a part of the series. probably 201-level, as you note.

        I am going to do 101 and 201 posts side-by-side, so that the more advanced readers don’t get bored. These are pretty quick and easy to do, so you should see more of them.