Making the AR-180B bolt hold open button

This post is about making the pivot button for the bolt hold open device for the AR-180B.


With no engineered drawings to work from the first item of business to take care of was making a drawing of the part with all the dimensions. To do this we used micrometers, feeler gauges and an optical comparator.


Using these measurement devices we were able to accurately measure theĀ  the complete button. We also used references from parts suppliers to verify correct dimensions for matching components.

Side-Mount External Retaining Ring (E-Style)

Black-Finish Steel, for 7/32″ Shaft Diameter

For Shaft Diameter 7/32
Fits Groove
Diameter 0.188″
Width 0.029″
Ring Size
(A) 0.437″
Thickness 0.025″
Additional Specifications Black-Finish Steel
RoHS Compliant

Snap rings into the groove from the side of the shaft. Their three prongs make contact with the shaft and provide a wider shoulder than other external retaining rings for a larger retaining surface. They are magnetic.

Black-finish and zinc and yellow chromate-plated steel rings have a minimum Rockwell hardness of C47.

Next came a solid model of the part.


2sA program was written with master cam to run on the cnc lathe.

This is the video of machining the part on the lathe.

This is an picture of the part as it comes off the lathe when the first op is completed.


A comparison of the original part to the newly machined one.

DSC_5689csWhat has been done so far with still more to be made.

DSC_5691csThe second op to come

5 comments to Making the AR-180B bolt hold open button

  • Does the part get chucked back up from the other end and the parted side finish machined to size?

    Great job so far and I can’t wait to see the completed kit.


  • Chuck

    That is op 2 and will be done this next week.

  • Jim Sturges

    I’ve worked many years a receiving inspector for a defense contractor. McMaster-Carr always has great parts, but sometimes has limited options. the “black finish” is probably Du-lite, similar to hot bluing. I know there are fasener parts houses in Sothern California that can supply E-rings and snap rings with dark gray Parkerizing. Unfortunately I can’t remember their names.

  • Kerwin Kerr

    I don’t need a bolt hold open stud. I’ve got one of those! Who’s got a bolt hold open for sale? My AR-180 is a Costa Mesa unit made by Stirling of England.

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