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:
- The rifle fires, the bolt carrier moves rearward, and the bolt begins to rotate open.
- Pressure in the chamber drops significantly, meaning the rearward force on the bolt also drops dramatically.
- 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.
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Authors: the Brass Valley 2019 crew.