Let's talk about MK760 extractors

[Jim in Texas]

I have been involved with MK760s for a few decades now and am bothered by the increasing lack of readily available spare parts.  The MK760 is a fairly simple design, so I decided I would try making my own spare extractor.  This is the tale of that adventure.

The factory extractor for the MK760 looks like it was stamped out of a sheet of steel – great for inexpensive mass production, but the resulting part is not quite flat and is not quite uniform across its width.  Additionally, I do not have the capability to make parts that way.  Instead, I opted to modify the design slightly so I could make the extractor on a mill.

Machining the new extractor also allowed me to make it slightly thicker than the factory extractor – I did not have to deal with the deformation caused by stamping.  I reasoned that the thicker extractor would also get a better grip on the cartridge case and would be less likely to tilt or cant in the extractor slot in bolt.  I did have to clean a little flashing from the extractor slot so the new extractor would operate smoothly.  

From reading reports of failed extractors, it sounded like chipping of the hook was a common failure mechanism.  I decided to do two things to try to address that.  First, I made the extractors from pre-hard steel (about 28-30 on the Rockwell C scale) and did not harden them beyond that.  I reasoned that this would be hard enough to prevent premature wear, but not hard enough for chipping to be a problem.  The pre-hard material also machines nicely.  Second, I made sure to incorporate a small radius in the underside of the hook to try to minimize stress risers in that area.

The machine work was fairly straightforward.  In the first 500 or so rounds, the resulting extractor has functioned 100% and I do not see any signs of wear.  

As I have previously mentioned, I am not a professional engineer or gunsmith.  Additionally, there are a lot of variations in the world of 76/760s.  These ideas may need to be modified, or they may not work at all with your particular firearm.  

Here are some photos of the new extractor alongside the factory extractor.  Thoughts?  Comments?  What changes would you recommend if I tried this again?

 

[StrangeRanger]

In general sharp inside corners are not your friend in terms of longevity.  They generate fatigue cracks even when lightly loaded  The one on the extractor hook itself needs to be there but the other two could easily be radiused on any future runs

[Jim in Texas]

Agreed.  That is why I put a small radius (0.016" radius) on all 3 inside corners.  I could have put a bigger radius on the back corners, but that would have required a tool change.  As I am an amateur with a manual mill, I wanted to avoid losing my Z-axis zero if possible.  Is a bigger radius on those back two corners worth losing my Z-axis zero?

[StrangeRanger]

It's hard to say but it probably won't make a huge difference. Only time and round count can answer that question

[johnsonlmg41]

The only one that would matter is the one at the end and you've got that covered.  Often what breaks them is out of index barrels and riding over the case head, not pulling on the soft brass, which is minimal force at best on open bolt subguns.

[Rekraps]

So, how does this extractor work with OEM 76 bolts? I know that the Andrey bolts had prorprietary extractors, so that is obviously out, but OEM S&W 76 bolts? Will it fit and work?

[Jim in Texas]

All my experience is with MK760s.  The bolts that I have access to are OEM Irvine bolts with the flat transition and that use the long extractor.  it fits them without issue.  I did make sure there no flashing left over from manufacturing in the extractor slot before I tried the new extractor.

I wonder which bolts have the curved transition and take the short extractors.  Could that have come first with the early tool room S&Ws?  Was the design then simplified to the flat transition in later S&Ws, which was then carried over to the MK Arms guns?

 

Edited April 28, 2025 by Jim in Texas

[Don.]

I think I might still have two failed extractors that I can get pictures of if that helps with your diagnoses.  One was the one that came with it.  The other is one that I made and ran to failure.  I can’t recall the round count, but it was probably less than 1K up to 2K?  I shot it a lot more back then than I do now.  The breakage could have been from a 7.62x25 conversion I made that I was running too.  Sorry, it was a while ago.

Your replacement looks great.  Run the hell out of it and track the round count.?

Edited April 29, 2025 by Don.

[Jim in Texas]

Don:

Yes, I would very much like to see photos of those two extractors that failed.  I think that would be quite helpful.  Can you also tell us more about the one that you made?  What material did you make it from?  Did you include a radius on the inside corner of the actual extractor hook, and if so, how big a radius?  Did you harden the extractor after making it, and if so, how hard?

Thanks

[StrangeRanger]

Don't know whether you've seen these but they might be useful in confirming your extractor hook dimensions

https://bobp.cip-bobp.org/uploads/tdcc/tab-iv/tabivcal-en-page28.pdf

https://i0.wp.com/www.theballisticassistant.com/wp-content/uploads/2021/01/9mm-Luger.jpg

[Jim in Texas]

Thanks.  Yes, those SAAMI drawings are a wealth of information.

 

[Rekraps]

17 hours ago, StrangeRanger said:

Don't know whether you've seen these but they might be useful in confirming your extractor hook dimensions

https://bobp.cip-bobp.org/uploads/tdcc/tab-iv/tabivcal-en-page28.pdf

https://i0.wp.com/www.theballisticassistant.com/wp-content/uploads/2021/01/9mm-Luger.jpg

You always have the "poop"!