Gas Trap M1 Garand

The original design of the M1 Garand as adopted in 1936 used a “gas trap” system instead of a gas port drilled in the barrel. This system used a type of muzzle cap and false muzzle to redirect gas into the gas cylinder in the short distance between the end of the rifled barrel and when the bullet left the muzzle. The system worked, but was not ideal.

Several problems were found with the gas trap system as the guns went into production. These included cleaning complexity, an unstable front sight, and a potentially weak bayonet mounting point. Most significantly, however, one rifle in testing had a screw work loose in the muzzle cap, which allowed parts to shift out of alignment and resulted in a bullet striking the gas plug and blowing the entire assembly off the gun. This led to a decision to redesign the gas system of the M1 to use a simpler gas port drilled in the barrel.

When this design change was made, 18,000 rifles had been completed and parts were made for an additional 33,000. Those guns were completed with the available parts, and the new gas system was used for all further production. Gas trap M1s are very rare today because the guns were updated to the new system when they were overhauled during WWII, and in 1947 the Army ordered all remaining gas trap rifles destroyed.


  1. Couldn’t the designers of gas trap guns like the gas trap M1, or the German G41 have taken a look at Lewis, Hotchkiss, and Colt “Potato Digger” machine guns, all of which has gas ports in the barrel, and had been around for decades already, served in World War One, and had tens of thousands of rounds fired through them? Wouldn’t a good look at well-worn examples of those guns have told them their concerns over a gas port in the barrel were a largely unfounded?

    • It’s prudent to note that firearms design was still in the developmental phase regarding what worked and what did not. While the example you listed worked well, designers were still trying to decide what was rock solid and what looked good on paper only.

      • “still in the developmental phase regarding what worked and what did not”
        Which is very obvious if you consider 1920s-1930s Soviet self-loading rifle.
        You might know that there were AVS adopted 1936 (design by Simonov) and SVT adopted 1938 (design by Tokarev), but there were many many other design, some can be seen here:
        There was design by Degtyarev produced in very limited number in early 1930s, but it proved to be outrageously unreliable; design: gas-operated, боевые упоры (I don’t know American parlance – same principle as used in DShK).
        There was design by Рукавишников in 1938, which was gas-operated, locked by tilting to left.

    • At that time, there was strong prejudice in military ordnance circles against gas systems that require holes drilled in the barrel.

      Never mind that the most successful gas-operated machine guns, the BAR, the ZB/Bren, and the Hotchkiss, all were made exactly that way. The ordnance bureaus just didn’t like it, and that was that.

      I suspect there was a good bit of “Not Invented Here” syndrome at work, there, as well. Gas trap was “good” because it was the “in house” design, not the work of some outside “inventor”.

      And of course, if developed by Ordnance personnel on Ordnance salary and working on Ordnance time, the concept was government property. Hence- no royalties to pay.

      Governments have in the past done stupider things for much less reason. And continue to do so today.



      • “no royalties to pay”
        Wasn’t classic gas-operated patented by someone at time Garand rifle was developed? Then this way would be way to go around patent fees.

        • The Browning, Hotchkiss, ZB and Lewis gas systems were all protected by patents. That pretty much covered the practical barrel-port gas-operation systems.

          There weren’t that many patents re muzzle gas-trap systems simply because most actual designers (like Browning) had tried them and abandoned the idea in favor of more practical systems.

          The ordnance bureaus forgot one of the basic rules of engineering;

          If Nobody Does It That Way, There’s Usually A Very Good Reason.



          • Aha! Last year I speculated that patent issues were to blame for the Bang rifle requiring a gas trap! I mentioned this during the discussion about a prototype Bang example. If Garand had used any gas port already in production, Lewis or Hotchiss company would have sued his pants off.

      • You can say they were “not up to date” or “current” with respect to new developments. Some habits/ways of tinking die hard.

  2. Also remember the Philippine division at Bataan had gas trap garands in it’s Infantry regiments- 31st US INF, 45th, 57th Inf(PS)

    • And they worked quite well, from many accounts.

      Saw this on the tube a while back. At the end of the old Mel Gibson/Sigourney Weaver movie “Year of Living Dangerously”, which was about Indonesia under Sukarno, Gibson’s character is in a cab going out to try to get an interview during the coup, and going thru a gantlet of soldiers. There’s a flash of one soldier holding an M1, and I’m pretty sure it’s a gas trap rifle, although I haven’t gotten a copy of the movie to go back and tell for sure. The movie was shot in the Philippines.

      But one thing’s for sure, the military didn’t destroy all of them. Most were converted to gas port rifles, but some original examples still exist.

  3. Great post today Ian. A big “thank you” to Julia Auctions for allowing disassembly of the gas trap. A neighbor across the alley was in WW2. Joined in early ’42. He and his buddies filmed themselves at the range one day(8 mm B&W). Bob demonstrates a quick field strip and reassembly of an M1. Its fuzzy but I’m pretty sure he’s holding a gas trap Garand. I’ve held one, I have books showing drawings but never saw the guts removed. Thanks!

  4. It would seem from the outset that tapping the gas after the bullet had left the barrel would cause the bullet to yaw due to the difference in pressures above and below the unsupported bullet, while perhaps having two tapped locations 180 degrees apart would have alleviated the pressure imbalance that would have otherwise resulted from having a non-concentric post-barrel chamber, and thereby resulted in less bullet yaw (if that makes any sense).

    • aa: I agree on the yaw part….but with each rifle a shooter could adjust windage at the rear sight. I think a bigger problem would be rezeroing after each cleaning – removal of the front sight post. Put together it’s easy to see why a change was needed.

      • The M1 barely holds zero as it is (at least by modern standards) due to the way the receiver beds down in the wood stock. You can zero your M1 one day and then shoot it again a few days later and it will be off zero even if you never field stripped it. If it’s a well fitted M1 with a stock made from a good cut of wood, it should be no more than 1-2 MOA off, but it’s not unusual for it to be 3-4 MOA off. The fiberglass M14 stocks were developed in the 60’s to address the issues of wood and humidity. It probably was not a big deal in field use for the average soldier, but it is an issue for match shooting both back then and now. The DCM was skeptical that a National Match M1 made sense, and the first ones were essentially carefully assembled and tuned battle rifles. The glass bedded stocks came later.

        I’m not sure how much that removeable front sight would have affected zero. The front sight sat in the same keyway on the barrel, so as long as they fit reasonably tight when new, I’d think it would have been fine. There is a good bit of slop on a gas port M1 gas tube anyway, but it attaches to the barrel by three spines, instead of one keyway. I haven’t noticed much of a difference in accuracy peening the splines on M1 barrels, so it was certainly acceptably accurate for field use.

        • The logical solution would have been to have windage and elevation adjustments on the front sight, as on most LMGs and GPMGs with quick-change barrels.

          Of course, Ordnance missed that one on the original M60 GPMG as well, not to mention doing silly things like mounting the bipods on the quick-change barrels and not attaching a carry/barrel-change handle to each tube.

          Making each barrel heavier than it needed to be, leaving the gun unsupported during a barrel change, and of course making handling a hot barrel a major PITA even with the legendary asbestos-lined “oven mitt”.

          Or as an old Marine gunny I knew once said, “If Ordnance designed something themselves, you can just about bet that it’s not gonna work right when you need it to”.



        • Re: Zero. In a well bedded M-1 rifle, the zero upon reassembly may be off by a small amount, but it will not be overly significant for any common field use. The exact same is true of any other common infantry rifle that is dismantled from the stock then reassembled. If the receiver screws are tightened exactly the same as prior to disassembly, the rifle will be very close to zero. In worn rifles, it is less likely that the rifle will be at zero when reassembled. This is not different from the M-1 Garand.

          For match use, any wooden stocked rifle will have to be rezeroed after disaasembly/reassembly. Even if glass bedded, rezeroing will be required simply because of the nature of match shooting.

          It is true that a less than secure front sight mount will affect on target results. A tightly fitted gas cylinder renders that a non-issue with the M-1 Garand. Peening the splines of the barrel will provide a long lasting tight fit. A saw cut gas cylinder is an excellent way to make sure the gas cylinder does not move. Of course best of all is a gas cylinder that is a driving fit on to the barrel. The above reflects merely my own experience shooting M-1 rifles over the last 30 years. YMMV. Sincerely. bruce.

  5. This gas trap on Garand is new knowledge for me. I thought it was gas piston via port operated right from start. In that sense and in comparison to Johnson rifle it was not as such a “breakthrough” or last shout of modernity.

    Thanks for showing it.

  6. does anyone know how they handled the problem of rifle grenades? did they not have them at this timeframe? I can’t see how you’d put a rifle grenade spigot on there.

  7. Your claim that after 1947 Gas Trap rifles were “destroyed” is absolute NONSENSE.
    The 1947 Manual merely states Gas Traps are not to be rebuilt or repaired AS Gas Traps. The word “destroy” ain’t in there!! That would make no sense as converting a Gas Trap to a Gas Port rifle is a simple barrel and parts swap.

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