AR-180B trunnion design

An interesting question was brought up by Rick at  a while back about making new AR-180B upper receivers. The actual receiver is not much of a problem however the trunnion is. The expense of making a new trunnion would be cost prohibitive so a different solution had to be found. In this case the thought would be to do what was done, or thought to be done by Armalite and that is using an AR-15 barrel extension pressed into a new trunnion block. So looking at a AR-180B we can see what the trunnion looks like in a receiver.


DSC_6119csLooking at the above pictures you can see the difference between a AR-180B and the AR-180  below.

DSC_6125csThe above picture shows a flush front end with the threads. While of the 180B it is recessed.



There are a number of differences. The original is forged and then machined. You can see how the boss sticks out in front of the bolt locking area.  While the 180B the rear of the trunnion is flush.

The reasons for these differences is because a AR-180b does use a AR-15 barrel extension pressed in. However there are changes to the barrel extension.

In this hidden view you can see that a AR-15 barrel extension would be to large for a trunnion as originally designed for an AR-180

AR-180B barrel extension shadowThis is a solid view of the same model. The barrel extension would get in the way of the recoil rods.

AR-180B barrel extension 3Even reducing the diameter of the barrel extension would not be enough to correct the problem. The extension has a diameter much larger then the inside diameter of the trunnion.

AR-180B barrel extension 2

AR-180B barrel extension 1

This can only work if the trunnion is also re-design. This is what Armalite did for the AR-180B.

AR-180B barrel extension 180B proper 1Instead of a rounded bottom they squared it off to allow more material to support the turned down barrel extension.

AR-180B barrel extension 180b proper 2

AR-180B barrel extension cut away

As you can see in this cut away the reduced size of the outer ring of the barrel extension.

In this picture you can see the hole for the guide pin on an AR-15 barrel extension.


9 comments to AR-180B trunnion design

  • I knew that they must have turned down the diameter but the flat on the bottom is a revelation.

    That solves two problems at once, room for the locating boss and it keeps the extension from rotating.

    Thanks for all your hard work.

  • I just re-read the article and I misstook the flat bottomed trunion for a flat bottomed extension.

  • Kerwin Kerr

    This is a good idea if it reduces production cost. Has anyone given any thought to my engineering change of converting the new prototype to take a STANAG magazine? One of the main reasons why the AR-180 was a failure was because it didn’t use STANAG magazines. The original magazine was actually a well thought out construction, it was just put on the wrong side of the gun!

  • Kerwin Kerr

    Correction! On my last sentence I meant magazine release!

  • Pete F.

    Interesting research on the barrel extensions. I have those parts but never looked at them in so much detail.

  • Dan E

    thanks for this, is very informative

  • Rufus

    Are there any dimensioned drawings for this 2 peice trunnion ?

  • Kerwin Kerr

    I would also advise putting a 1913 Picatinny rail on top of the receiver. If it’s aluminum you can you can rivet it to the top of the receiver. If its steel then I’d have it either spot welded or heliarc welded to the receiver. Using the old style AR180/18 scope mount rail is not a good idea.

  • Charles Moeller

    When I tried to remover my barrel the barrel extension turned instead of the barrel unscrewing. What should I do now?

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