Mk760 Bolt
- Thread starter navgunner
- Start date
MontanaRenegade86
UZI Talk Supporter
Assuming I measured correctly...
Bolt OD: 1.121"-1.122"
Tube ID: 1.151"-1.152"
Bolt OD: 1.121"-1.122"
Tube ID: 1.151"-1.152"
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
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
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What’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
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Irvine, Low mileage MK760 (bolt shows minor wear, park is still mostly there on the contact surfaces).
S prefix (factory SMG, at least was originally intended to be one).
Bolt outside diameter 1.1260
Tube, inside diameter 1.1460 (this was hard for me to measure).
Used Chinese calibers, so take that into consideration.
Bolt face (guessing flat)
S prefix (factory SMG, at least was originally intended to be one).
Bolt outside diameter 1.1260
Tube, inside diameter 1.1460 (this was hard for me to measure).
Used Chinese calibers, so take that into consideration.
Bolt face (guessing flat)
MontanaRenegade86
UZI Talk Supporter
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.
MontanaRenegade86
UZI Talk Supporter
Andrey claimed his curved transition bolt worked in all S&W 76s and MK760s.
I am hopeful (and confident) that Navgunner will make us a great bolt than can be used in either model. The one I am sending is almost there, but not quite.
I am hopeful (and confident) that Navgunner will make us a great bolt than can be used in either model. The one I am sending is almost there, but not quite.
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.
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?
MontanaRenegade86
UZI Talk Supporter
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?
I think your bolt is the flat transition like all of mine are.
MontanaRenegade86
UZI Talk Supporter
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.
Jmacken37
Mr. Miata, UZI Talk Life Member,
I remember years ago, when speaking with Scott Andrey, he had mentioned changing the orientation of the extractor in the bolt to avoid the S&W76's tendency to break extractors. If you are designing a new bolt anyways, this might be something to look at...
navgunner
UZI Talk Life Member
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.
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.
MontanaRenegade86
UZI Talk Supporter
Right on.
Moving the extractor position would mean a trunnion modification to run the new bolt.
ferndog1
UZI Talk Supporter
Burgess SW76 article...
navgunner
UZI Talk Life Member
I’d love to see any data that supports Jim‘s claim that relocating the extractor makes it a stronger system. It’s purely my speculation, but I doubt if there was ever a FEA (finite element analysis) done by engineers to quantify improvement.
Metallurgy has a lot of components, but two of the biggest are toughness versus hardness. Furthermore, engineers have to find an appropriate balance in systems (and there’s always concessions made) due to time, cost, available materials, current technology, manufacturability, etc.
All of the original factory S&W extractors that I’ve seen were stamped and I can only assume they didn’t have a secondary heat treat. Modern technologies allow manufacturers to pre-harden material and precision cutting equipment to mfg. extractors without disturbing the metallurgy (EDM is one an example). In short, materials and manufacturing processes are better than they’ve ever been, and I would argue that there’s no need to change the geometry or location of the extractors.
Like I said above, these are my thoughts, and of course I could be wrong, but to properly assess, a qualified individual/team would need to do testing using the scientific method as its core (not conjecture or sales pitches).
If anybody has data, I’d love to see it. Please share.
MontanaRenegade86
UZI Talk Supporter
Now I've seen what a curved transition bolt looks like.
@Rekraps over at Machinegunboards has some good shots of his Andrey bolt.
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