Operating Systems 301: What Is Underlug?

Note: In this article, I call this mechanical feature “underlug”. However, this is an error. Several friends of mine and I have been discussing the mechanics of firearms operation for close to a decade now, and we misremembered the term “underslide” from a book by Brassey’s as “underlug”. More details on the error are available in the comments. Regardless, “underslide” is the proper term for this principle, not “underlug”.

When a firearm unlocks, even if the bullet has already left the barrel, some residual pressure remains in the chamber, forcing the cartridge case walls out against the chamber, and causing resistance against the extraction of the spent case from the barrel. Within a single instant, the pressure in the barrel is relieved and the case walls rebound elastically away from the chamber. In a manually operated gun, the reaction of even the quickest shooter is too slow to operate the gun before the pressure in the barrel can drop, so this residual pressure does not present a problem. However, in automatic weapon design the weapon’s mechanism virtually by necessity must begin to operate while pressure in the chamber is still high, and this presents the problem of how to provide energy to the mechanism while there is ample pressure to do so, yet still extract the cartridge case when the pressure drops to a low enough level to allow the case to easily leave the chamber without risk of shearing a rim, or worse.

There are several inventions which solve this problem, including the expanding gas system of the AR-15 and John Pedersen’s cartridge waxing process, but one of the most mechanically sound ways is called underlug. Underlug is a special dimensioning of the cam surfaces on a locked-breech firearm which give the actuating piece space to move before it actually contacts the locking element and unlocks the firearm. As an example, below we can see the underlug on the cam track of an M1 rifle’s operating rod and how it interacts with the unlocking stud on the bolt lug, at the timestamp 5:29 in the U.S. Army video below:

You can see from 5:29 – 5:36 how the operating rod is able to move to the rearward significantly (in reality, roughly 3/8ths of an inch/9mm, although significantly exaggerated in the illustration above) before it encounters the locking stud and forces the bolt open. This delay in unlocking not only gives the operating rod a running start to unlock the rifle, it also allows pressure to drop in the chamber before the extractor is forced to pull the case out, easing extraction.

In the locked position, the bolt is rotated into position and the operating rod is fully forward:


The operating rod is able to move back slightly (~3/8″), but the bolt stays in the locked position and does not yet start to rotate:


Now the bolt has rotated out of alignment with the locking surfaces in the receiver:


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.


  • plingr2

    So, underlug is the movemant of bolt carrier, before bolt start moving and unlocking mechanism ? (Sorry for my eanglish)

    • Ken

      More or less, the bolt carrier is built to allow for such a movement,

    • Riot

      Its like having one rope resting slack in a pulley system.

  • Isaac Newton

    Why is this concept called underlug? I would have thought it would be called cam dwell zone/interval.

  • PK

    “What is underlug?”

    Firearms version of underboob?

    More seriously, I’ve never heard this referred to as “underlug”, except by you. Where did you first come across the term, anyway? I looked through Chinn’s work, Rheinmetall’s handbook, the 1968 tech notes from USA WC… nothing.

    • Justin Galt KRG

      I wonder that myself. I’ve always seen/referred to it as “carrier free travel”. At any rate, great articles you’ve been posting Nathaniel!

    • ostiariusalpha

      Obviously break actions and revolvers have components called underlugs, but the source of Nathaniel’s use of the term is a bit of a mystery to me as well.

      • PK

        I meant in this context, but yes, the underbarrel lug or underlug is present on revolvers and some break actions, as well. Good point.

        In this context, all I’ve found is this post… and the previous post from Nathaniel about the Colt LMG.

    • Alright, so once people started asking this question, it occurred to me that I didn’t have a good answer. A couple of buddies of mine and I have been farting around with small arms design theory for a few years now, and we’ve been using “underlug” for ages, so I asked one of them about it. He says he got it from Brassey’s Essential Guide to Military Small Arms: Design Principles and Operating Methods, which is available for the low low low price of $791.35 on Amazon.

      So there you have it.

      • PK

        Nope, that’s not it. The Brassey’s guide calls it “underslide”. First use of the term is on page 102. Let me know if any of you recall where “underlug” came from, I’m always on the hunt for new sources.

        • Iiinteresting. That suggests “underlug” is a corruption coming from my buddy. And now I’ve reproduced it here. Hah!

          • PK

            Hey, it happens. But if you’ve got a copy handy, it’s the first two sentences on the paragraph, under figure 7.15 on page 102.

            “The shape of the control cam groove provides a small amount of underslide of the carrier during unlocking, i.e. a short free path of the carrier until the start of unlocking. The amount of underslide is chosen to allow the pressure to drop in the barrel.”

          • I don’t have a copy. It’s ludicrously expensive. My buddy got it on inter-library loan.

          • PK

            Fair enough. Keep your eyes open for a copy, I couldn’t have paid more than a couple hundred for mine.

          • Gorilla Biscuit


          • Thanks for the scan!

            Well, this does put me in a weird position, as I have been using that term for this concept for going on a decade now.

            What do you all think? Edit the article to correct the terminology, with a footnote about the error, or what?

          • Gorilla Biscuit

            I say just keep using it.

          • PK

            Correct it, or explain “also called underslide” and reference the Brassey’s guide? Adding a new term from whole cloth might make searching for it later frustrating to those who want more information, so the more leads you leave, the better.

          • ostiariusalpha

            You can’t exactly say “also called underslide,” as that is what it is actually called, whereas “underlug” is simply a mistake. He should change it to “(bolt carrier) free travel” since that’s what it’s called in modern texts, with a reference to underslide.

          • PK

            Thick books sure don’t scan well. I keep thinking of doing that to a few sections for reference, but it just never turns out very well. I’m glad I’m not the only one here with a copy!

          • Jeremy Nettles

            I learned in grad school from one of my professors that the best way to get a scan from a thick book (or a book that’s thick in comparison to its other dimensions, at least) is to take a razor and slice through the backing material between two signatures (the folded collections of pages) in the middle of the book. This frees up the rest of the binding to lie flat on a scanner surface without fighting against the spine for space to do so. Of course, this requires you to cut open your book, so it kind of depends on just how badly you need to scan it!

            (This assumes that it’s a legitimate binding and not card stock and glue, but not many of the latter sort of books are worth scanning, are they?)

  • The_Champ

    This term is new to me as well. A quick search only references revolvers.
    Do you know it’s origins Nathaniel, or have reference material that uses the term?

  • disqus_yU5rfSFNc3

    Guys, it’s timing, come on you all know what’s going on here. We need time to go by in order to allow chamber pressure to go down to a safe point so we can unlock the breech. How do we do this??? By leaving a space between operating parts or time before contact. Or we would be timing the action to operate correctly. Timing in any operating system is created by putting exact distances between operating parts, or time before actuation. Look at the AK operating system, watch how far the bolt carrier travels before the bolt starts to rotate. I always heard the term timing the action, never heard of an underlug except on revolvers.