This is fantastic information. Thank you for sharing!irvine, ca gun
bought NIB in 1990 single owner
bolt looks to be 1.125"
receiver is broach cut I assume to let burnt powder and such debris as would accumulate escape
receiver measures 1.150"
at broaches measures 1.180" so broaches total is 0.030 or 0.015 depth at each broach
gun was a "C" prefix destined to become a carbine until the May 19th rush.
When gun arrived in arkansas first thing john norrell did was test run it as he had previously had 12 guns himself that each had to be worked on to run---he had a friend's sw76 to pattern from to spec each MK760
this gun (purchased be me from roger small at automatic weaponry in nashville, tn.) ran fine from day 1 with no fixing or tinkering at all
What’s your prefix?Assuming I measured correctly...
Bolt OD: 1.121"-1.122"
Tube ID: 1.151"-1.152"
S005X - early Irving MK760What’s your prefix?
Irvine gun?
Thanks for posting.
Could we get pictures of the bolt faces or mention which bolt face you have?
Maybe, we all can find out if there’s any rhyme or reason to why certain guns have one of the two types of extractors.
Link below for bolt face info.
http://www.samachine.com/76 Ext.htm
S005X - early Irving MK760
Left-to-right: S&W 76 bolt from U4XXX, unknown bolt, MK760 bolt from S005X
I suspect the unknown bolt is a OEM S&W bolt, but some of the geometry is different. It was NOS until today, now it's been test fired. It fits and functions perfectly in my S&W 76.
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I’ve never tried swapping extractors between the two.Huge thanks!
The bolt face looks the same as mine.
Do you agree?
Kinda hard to tell because the underside photo is taken at an angle (see red arrows, in the photo below).
Maybe some good news. I’ve guessed for some time that the factory guns (S prefix) used the same bolt. Does your sw76 extractor fit and work in your MK760 (S) bolt?
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Good to hear.I’ve never tried swapping extractors between the two.
I think your bolt is the flat transition like all of mine are.
I don't think I've ever seen one of the curved transition bolts.Good to hear.
I think RSSC is going to check his as well.
Thanks again.
Right on.That’s something that Burgess did with his bolts but I couldn’t tell you that it actually made a difference beyond conjecture.
Honestly, I would make a reproduction bolt as close to factory dimensions as possible because I don’t have an engineering package or someone that was there during the design process to guide me. It’s impossible for me to know all of the “whys” that went to Smith & Wesson’s production decisions and with merely dozens to be sold (not hundreds or thousands) it’s ultimately not worth my time to deviate.
Burgess SW76 article...That’s something that Burgess did with his bolts but I couldn’t tell you that it actually made a difference beyond conjecture.
I don't think I've ever seen one of the curved transition bolts.
Well, I guess I saw the one on Andrey's website, but it didn't have a good view of the bolt face.