The Locking Lugs of The Kalashnikov Family of Firearms, and Their Changes

The Kalashnikov family of firearms is host to a significant amount of clever features. From machining techniques to overall concept, Mikhail Kalashnikov and his team were incredibly bright.

One of these features that has grabbed our attention as of late is the bolt and trunnion locking lugs. It is not entirely uncommon knowledge among gunsmiths and tinkerers that the lugs on the AK family of rifles are swept at an angle (rather than flat like many other rifles), but we have discovered this topic is far deeper of a rabbit hole than it seems at first glance.

The main locking lug on an AKM bolt
Featuring a serial number that was ground off by a previous owner

The AKM has a lot of features designed to promote reliability – one of which is the helical (angled) locking lugs. Some of the benefits include:

  • Aided primary extraction, the camming action helps break the friction of a fired casing that has expanded and stuck to walls of the chamber
  • An angled locking surface has more surface area than a flat one of the same outer dimensions – resulting in more service life before losing headspace.
  • More forgiving factory headspacing. When the factory sets the headspace, they can set it tighter than you otherwise could on a flat lug, again, resulting in a longer service life.
  • A “wiping” action – as the bolt returns home and rotates into battery, the leading edge of the bolt can wipe debris out of the way

Now, here is our recent interesting discovery.
The angle of the lugs on either side of the bolt changed through the history of the AK. The earliest blueprints we have are those for the Type 3 milled receiver AK, so we’ll start there. I suspect the T1 and T2 were the same, but if anyone has one, I’d love to hear from you.

The lugs on the Type 3 AK are defined as a helix, having a diameter of 25.86mm and a pitch of 3mm. Meaning, if you were to bore a hole and tap it with a tap that was 25.68 x 3mm, then cut away the rest to look like the front end of the milled receiver, you’d have your locking lugs in there.

Front view of the Type 3 locking area

Next up: The AKM.

They changed the angle of the lugs here, from the 3mm pitch (~2.11°) to a straight 4.5° on the right side lug. The left side lug remains 3mm pitch.

Why this change?

Well, we can’t exactly ask the designers, but we can make an educated guess. We believe this change was made to better handle intrusion of dirt and debris.

With the smaller left side lug being tucked away inside the rifle, further away from the ejection port (intrusion area), it has less of a chance of encountering dirt than the right side lug does. Given that the left side lug is ~2.11°, and the right side is 4.5°, here’s what happens:

  1. The rifle fires, the bolt carrier moves rearward, and the bolt begins to rotate open.
  2. Pressure in the chamber drops significantly, meaning the rearward force on the bolt also drops dramatically.
  3. The instant the bolt rotates, the right side lug is no longer touching anything. The left side lug, being a shallower angle, remains engaged.

But why? We believe the answer to be friction. The bolt now only has to overcome friction on the one, much smaller lug. This should, in theory, “sap” less energy from cycling, leading to more reliable operation.

Lastly, the AK-74 and 100 series.

Unfortunately, I do not have the blueprints for these, but I do have a significant sample size of parts, and measuring equipment. All bolts measured come in at slightly over 2 degrees, from which we can reasonably conclude they returned to the 3mm pitch lugs.

It seems strange they would begin with 3mm pitch on both lugs, then change to 4.5° on the AKM right side lug, then return to 3mm pitch.
This is likely something to do with simplicity of manufacturing. They likely found the reliability of the symmetrical lugs to be satisfactory enough to abandon the 4.5° right side lug.

AK-74 bolt (top) with a shallower angle than the AKM bolt.

Thanks for reading!
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Authors: the Brass Valley 2019 crew.