Author Topic: SKS Barrel/Receiver Index Numbers Identified and Verified  (Read 26087 times)

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Offline Justin Hell

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #60 on: December 02, 2017, 12:35:01 PM »
I got to know.. think1  in a mass produced weapon, thats minute of man, designed for basic infantry with minimal training and maintenance and going to be used in harsh conditions, did a curve or barrel bow on the order of .001 make that much difference. Seems like a lot of effort for minimal gain, a target rifle, a sniper riifle yes, there would be a gain, but not a basic going to have the snot beat out of it and shot mercilessly on the battlefield, with a high probability of being lost in a ditch, in a pool of bodies rusting or some other fate.

The accuracy or durability gain would be minimal, if not a nil factor for the time and effort investment put into the task.

That monkey has a valid point right there.

Offline Phosphorus32

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #61 on: December 02, 2017, 01:04:55 PM »
Thought on terminology.

I think it important to point out that, in modern firearm manufacturing the term "barrel indexing" refers to the process of determining the curvature of the bore and then indexing/aligning that curve with the vertical axis of the receiver-- with the curve opening up with the muzzle oriented either at 6 or 12 o'clock.

I think this process is a fairly recent development in the construction of high accuracy long range precision rifles.

But, in the initial "arsenal re-barreling" thread over in the Russian sub-forum, the term "barrel indexing" led me to some confusion in fully understanding the processes that were being described.

I realized finally that the indexing being referred to, likely had nothing to do with the alignment of bore curvature and vertical axis of the receiver.

Is vertical alignment of the barrel curvature possibly an aspect of the numbered pairings? And if not, might a refinement of the terminology be in order to mitigate the potential for confusion?

A to Q1: No.
A to Q2: No.

Offline newchi

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #62 on: December 02, 2017, 07:12:30 PM »
Someone has to sight it in anyway

Offline Murray

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #63 on: December 06, 2017, 11:29:24 AM »
Sure hope there's a little popcorn left in y'alls buckets.
Bad things happen when I start thinking so I'm likely to just embarrass myself but a thought occurred to me about this numbering system. No doubt there is a specific purpose, but here's the deal. (actually first of 3 including BBs "bow")

The "thread pitch" (or "space" between each thread) is 1.5mm. That means for every revolution of the barrel, it moves 1.5mm, or .059". So divide that .059" by 36 numbers, and that means they inspected/marked them with difference between each number at just .00163888". Wow, one and a half thousandths of an inch difference?!

Type of material can enter the picture here, but varying amounts of "crush" are accounted for when machining and measuring, in some cases the thread shank length, but more often and more importantly, the headspace and any "indexing" involved. BBs contention "barrel bore bow" might have been involved is highly doubtful, as this would likely only apply to guys at a bench with their ammo being kept at a constant temp in a cooler, until they load and shoot.  ::)

At Alpha Arms, we would machine the barrel, allowing that .002" of "crush" would occur with the torquing of the barrel to receiver, meaning chamber was .002" too deep, until barrel was actually screwed on. So far, all these facts fly in the face of inspecting and marking every .0016" difference. If it was only the numbers 0,2,4,6 etc, and .003" and 20 degrees difference between them, then it would seem a little more logical.

And to add, old Springfield 1903's must have been manufactured with at least .005" crush, as they are torqued on the receivers tighter than any other gun we've ever worked on. In some cases, the "take-off" barrel has to be machined in front of the receiver, just to relieve pressure and get them apart.

(3rd deal) Now we know 'Smiths are all crazy... but got an old retired 'Smith buddy that claims he has a video of Chinese SKS manufacturing, with English speaking voice giving descriptions, and while I'll reserve final "say" until it's been viewed, he contends it shows a round barrel being screwed on to the receiver, then both mounted in a mill vise and extractor cut is very next operation. (important fact...this removes the need to install barrel, mark for extractor cut, remove barrel and machine, then re-install the barrel.) That's always been our method as there doesn't appear to be room for normal machining operation that would do this without machining the receiver a little. (sure never seen that)

 It's actually a 100 mi drive for me to go see the old AK/FN/HK/AR/Suppressor manufacturer/dealer buddy from the old days, (and he has cats) but I'll try to get a copy of it asap. May even ask him to snail mail it to me as he's the most 'puter illiterate 70 yr old I know.

With "pinned barrel" SKSs, I can see barrels being manufactured complete with all attachments already on it before attaching to the receiver, but threads muddy up the water. If the video shows machining of the extractor cut after barrel is screwed on, it only makes sense they would also then drill gas port hole, machine top and bottom side flats, then install all 4 barrel attachments.

But that leaves us asking...then what are the numbers for!? And likely has good old LC chomping at the bit to come back over and kick my arse! Yes, he was here and very well prepared for the testing, (kudos dude!) and the barrels were swapped and both indexed near bottom dead center, instead of top dead center.

One last thing today...Having removed quite a number of barrels from SKSs, I can say the amount of torque required to remove them varies between guns, even in a group with the same country of origin. And if numbering system was intended to match indexing of threads, the amount of torque used to "set" the barrel, should have been something they paid attention to. So the only thing that would cause us to be fully on board, would be if it was required to drill the gas port hole before chrome lining the barrel, and index numbers were just to get gas port at top center. Then it would all make sense.




Offline Loose}{Cannon

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #64 on: December 06, 2017, 11:52:17 AM »
Interesting!

But I have to say, it really all comes down to the video.  Most interested in seeing it asap.   thumb1

      
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Offline Phosphorus32

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #65 on: December 06, 2017, 12:39:24 PM »
I could be missing something too, but here it goes. I would assume the recess for the extractor is cut on the face of the chamber before installation of the barrel, so that necessitates that the receiver and barrel be indexed correctly.  The 0.0016" per 10 degrees seems like about the right amount of precision to allow for that much crush during torquing in that case.

Offline Loose}{Cannon

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #66 on: December 06, 2017, 12:52:51 PM »
If you really wanted to get anal about how this process was executed. We witnessed the barrels mating angle prior to torquing to be in the neighborhood of 20 to 30 degrees. Not only would they have clocked the timing of the male and female threads to establish the index point, they also would have either advanced the index value of the barrel jig, retarded the index value of the receiver jig, or both in order for the barrels to time somewhere between 20 and 30 degrees EARLY and that remaining amount of rotation to index is all torque.   thumb1


They literally could have added as much crush/torque as they wished.
      
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Offline running-man

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #67 on: December 06, 2017, 12:53:01 PM »
Good stuff Ben.   thumb1

I'd be very interested to see what kind of setup allowed the extraction port cut while threaded into the receiver.  I can see how the upper radius could be formed by the mill, but that lower radius in such close proximity to the receiver wall I'm just not seeing how to do it quickly and efficiently w/o occasionally kissing the sidewall of the receiver.



I have a few photo of early Russians that almost seem to have the lower radius under the lip of the receiver wall.  Maybe they did it different than the Chinese?  Good stuff all around!  thumb1





      

Offline Greasemonkey

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #68 on: December 06, 2017, 01:19:36 PM »
Now coming from someone who has rebuilt countless engines...

Quote
Having removed quite a number of barrels from SKSs, I can say the amount of torque required to remove them varies between guns, even in a group with the same country of origin.

This sounds odd.... sounds as if there is possibly no set torque value. Why the extreme variance....in manufacturing, torque has a spec for a reason.. Sounds like what were they using... the German torque value.....guten-tight and were just lining the marks up.

What I'm saying is.... when I rip out all 26 headbolts from a Cummins ISX, generally there is not to much difference between them when loosening, because they got torqued to a specific value when they were installed....goes like this.....150ftlbs, 300ftlbs and then pull your guts out and twist them another 90degrees. So.....if one is loose......ringy dingy....there an issue, might even be why your pulling the head. Every engine Ive done....Detroit 60, DD15, Mack E7 and E9, Caterpillars, Volvos... all the bolts for the respective engine pull about the same during removal.. unless one is broken, stripped, corroded, etc....

Their torque spec sheet could have read..
if SKS A needs 100ftlbs to line up...Pass
if SKS B needs 200ftlbs to line up...Pass
if SKS C needs 10 ftlbs to line up... FAIL...whoa...hold up start over..
if you rip out threads...you die!
As long as it's guten-tight..... tight....it's good, I bet they knew how far they could go to thread destruction. They just didn't want it loose. Too much torque is as bad, if not worse, than too little. I wonder, was it dry torque, or oiled torque, that can change values as well, a simple drop of oil can alter the final torque.

And either way.. the theory worked..  both were 180 degrees off. :)
« Last Edit: December 06, 2017, 01:30:31 PM by Greasemonkey »
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Offline Phosphorus32

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #69 on: December 06, 2017, 01:59:30 PM »
If you really wanted to get anal about how this process was executed. We witnessed the barrels mating angle prior to torquing to be in the neighborhood of 20 to 30 degrees. Not only would they have clocked the timing of the male and female threads to establish the index point, they also would have either advanced the index value of the barrel jig, retarded the index value of the receiver jig, or both in order for the barrels to time somewhere between 20 and 30 degrees EARLY and that remaining amount of rotation to index is all torque.   thumb1


They literally could have added as much crush/torque as they wished.

The crush was a side point, just seems like 10 degrees provides about the right amount of crush, as you've pointed out before.

Mainly, I'm a bit skeptical that the extractor recess is cut after the receiver and barrel are joined on the Chinese, Russian and Romanians. If they are cut afterward in some countries, maybe the gunsmith friend saw a video that was mislabeled as Chinese SKS manufacture but was actually Yugoslavian SKS manufacture and that explains why the Serbs didn't use the 0 to 35 numbering method. Do the Yugoslavian or other SKS barrels on the market have an extractor cut?

Offline Greasemonkey

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #70 on: December 06, 2017, 02:16:17 PM »
If you really wanted to get anal about how this process was executed. We witnessed the barrels mating angle prior to torquing to be in the neighborhood of 20 to 30 degrees. Not only would they have clocked the timing of the male and female threads to establish the index point, they also would have either advanced the index value of the barrel jig, retarded the index value of the receiver jig, or both in order for the barrels to time somewhere between 20 and 30 degrees EARLY and that remaining amount of rotation to index is all torque.   thumb1


They literally could have added as much crush/torque as they wished.

The crush was a side point, just seems like 10 degrees provides about the right amount of crush, as you've pointed out before.

Mainly, I'm a bit skeptical that the extractor recess is cut after the receiver and barrel are joined on the Chinese, Russian and Romanians. If they are cut afterward in some countries, maybe the gunsmith friend saw a video that was mislabeled as Chinese SKS manufacture but was actually Yugoslavian SKS manufacture and that explains why the Serbs didn't use the 0 to 35 numbering method. Do the Yugoslavian or other SKS barrels on the market have an extractor cut?

More importantly.... would China actually allow filming of their weapons building program or outside filming in a military weapons facility? Even if it were commercial made SKS, it was possibly still a super secret state run weapons facility in their eyes. Everything is a big ancient Chinese secret.

And yup... Yugoslavia used extractor cuts.
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Offline Loose}{Cannon

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #71 on: December 06, 2017, 02:17:29 PM »
The nos yugo barrels have no flats, no extractor cut, and zero inspection stamps of any kind.  Im not even convinced they are complete units. 

When we reinstalled the correct barrels to their respective receivers, I witnessed about 20 degrees of torque to index on one, and about 30 degrees on the second.  If you have a worker thread a barrel into the jig and the pre applied timing mark at 3 o'clock (witness mark) lands on 13.5 and they call it a 13 while a dif worker calls it a 14. THEN you apply the same +/- scenario to the receiver using its male jig........

Expect a difference of torque to index from one gun to another.     wink1
      
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Offline Loose}{Cannon

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #72 on: December 06, 2017, 11:27:38 PM »
Quote
The "thread pitch" (or "space" between each thread) is 1.5mm. That means for every revolution of the barrel, it moves 1.5mm, or .059". So divide that .059" by 36 numbers, and that means they inspected/marked them with difference between each number at just .00163888". Wow, one and a half thousandths of an inch difference?!

Bingo....  Pretty amazing what you can do by dividing 1.5mm into a rotational angle of 360/10.  Thats the entire purpose of clocking the first thread with a scaled jig.   As a few of us have experienced, a few thousands off the receiver face or lug face makes a big difference on where the barrel will ultimately mate and index after torque.  It's rather ingenious!


Quote
At Alpha Arms, we would machine the barrel, allowing that .002" of "crush" would occur with the torquing of the barrel to receiver, meaning chamber was .002" too deep, until barrel was actually screwed on. So far, all these facts fly in the face of inspecting and marking every .0016" difference. If it was only the numbers 0,2,4,6 etc, and .003" and 20 degrees difference between them, then it would seem a little more logical.

Not sure how much 'crush' would come into play on an sks which is headspaced after the fact via the bottom rear of the bolt.  The main objective would be to index the barrel without over/under torquing.  As I explained, the jigs reference marks can be laid out advanced or retarded rotationally clockwise or counter clockwise to allow the barrel to mate with the receiver a set degree before index.  They could have it 10, 20, 30 etc and the remaining rotation to index the barrel on the witness marks will be a controlled amount of torque. 



Quote
And to add, old Springfield 1903's must have been manufactured with at least .005" crush, as they are torqued on the receivers tighter than any other gun we've ever worked on. In some cases, the "take-off" barrel has to be machined in front of the receiver, just to relieve pressure and get them apart.

Not sure what the 'crush' on a 1903 bolt action has to do with a communist semiautomatic with a drastically different design and form of headspacing?  The 'crush' of the mating material is simply a result of the torque applied when indexing the witness marks, which I believe can be easily controlled, and headspaced after the fact via the bottom rear of the bolt.  I guess I'm failing to see how the amount of crush generated when the barrel is torqued to index suggests they 'didnt' use the numbers in the fashion our theory suggests.   :-\   In my mind, these numbers ARE a way to control the amount of torque to index and subsequently the amount of crush that comes with it. 



Quote
(3rd deal) Now we know 'Smiths are all crazy... but got an old retired 'Smith buddy that claims he has a video of Chinese SKS manufacturing, with English speaking voice giving descriptions, and while I'll reserve final "say" until it's been viewed, he contends it shows a round barrel being screwed on to the receiver, then both mounted in a mill vise and extractor cut is very next operation. (important fact...this removes the need to install barrel, mark for extractor cut, remove barrel and machine, then re-install the barrel.) That's always been our method as there doesn't appear to be room for normal machining operation that would do this without machining the receiver a little. (sure never seen that)


We would all love to see this video to see for ourselves.  You can hopefully understand the skepticism that comes along with something of this nature with so many variables. 

If our theory is correct, they wouldn't be removing the barrel to cut the extractor either..... Its already there when the barrel is installed.   thumb1


Quote
But that leaves us asking...then what are the numbers for!? And likely has good old LC chomping at the bit to come back over and kick my arse! Yes, he was here and very well prepared for the testing, (kudos dude!) and the barrels were swapped and both indexed near bottom dead center, instead of top dead center.


Laff Ben....  I may come back over there, but it would be to bring another set of rifles.  These having 9 numbers apart that would index 90 degrees from 12 o'clock.  In fact, the last test one barrel was 180 past index and the other was 180 pre-index, so a new test having 90 degree difference should land one at 3 o'clock and the other at 9 o'clock.  Now thats a test to be able to call out two separate orientations on one swap! 

We swapped two and torqued one and it wasn't near bottom dead center, it WAS bottom dead center.  The other would have been also had we fully seated and torqued it.   thumb1


Quote
One last thing today...Having removed quite a number of barrels from SKSs, I can say the amount of torque required to remove them varies between guns, even in a group with the same country of origin. And if numbering system was intended to match indexing of threads, the amount of torque used to "set" the barrel, should have been something they paid attention to.


Lots of variables come to mind... Ones like corrosion, but I would completely expect deviations within a certain torque parameter using our theory method, so I do not see the relevance. 


Quote
So the only thing that would cause us to be fully on board, would be if it was required to drill the gas port hole before chrome lining the barrel, and index numbers were just to get gas port at top center. Then it would all make sense.

I don't think the gas ports are drilled prior to chroming. With our theory, these numbers are not to align the gas port.... They are to rotationally align the lug flats, and the extractor cut and control the torque on index.  Ready made barrels and receivers numbered for assembly regardless of the assemblers IQ or location on the planet and basically zero milling required for mating. 


The very fact we were able to call out the exact orientation of two barreled actions based spicifically on their index numbers and had them index exactly where predicted is proof of what these numbers are for.   But I'm ready to do it again if need be.   thumb1
« Last Edit: December 07, 2017, 12:31:42 AM by Loose}{Cannon »
      
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Offline Greasemonkey

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #73 on: December 06, 2017, 11:35:51 PM »
Ok... maybe I prove something....maybe not.. but :)  Extractor cuts after the barrel install  nea1 ....well lets look and see if this is possible.  I pulled a few SKSs out, my ice cream melted and the X-Files was paused..  cry1


a Chinese /636\... the extractor cut is dead even with the receiver ledge..



A '71 Albanian... the bottom of the extractor cut is just a hair below the receiver ledge think1



Huuummmm, well, what say you.. thats some fancy machine work to cut that extractor groove after install... a '50 Russian... the mother of all SKSs



Not that this one matters.... this is a 416 PS pressed barrel... they was hitting a lick on lining this one up..




Now everyone is so concerned with Yugoslavia.... how they do it without numbering this or that, popping tags on barrels, binning them...yadda yadda..Guess no one wants to go look and compare them chuckles1 So......... THEY, Yugoslavia did the extractor cut after the barrel install..on M59/66s

Notice the extra cut in the receiver where the extractor cut ends and the receiver ledge begins.. they machined the extractor cut and over into the receiver. if China, Romania or Russia did this....like RM said, they would hit the receiver... and Yugoslavia did  thumb1 they in fact machined a little away cutting the extractor groove..

Funny... I've had this bastard for almost 20 years and never noticed... it's an "I" series M59/66





For gits and shiggles....this is a "C" series M59.. no receiver/extractor cut like above.. So possibly Yugoslavia modified their process somewhere between the "C" and "I" series




 
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Offline Greasemonkey

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #74 on: December 07, 2017, 01:29:11 PM »
Two more I dug up from random places in the house..  chuckles1

[0138] short lug



Type 84 /106\ short lug



Seems finding an extractor cut out thats cut below the receiver isn't hard...

Disclaimer..... yes..I realize they are greasy, slimy and nasty..just the way I like'em. I don't shoot them anyway. thumb1
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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #75 on: December 07, 2017, 01:43:46 PM »
Not sure what's in this video nobody has seen, but these cuts going below the receiver line highly discredits is validity before even being seen.
      
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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #76 on: December 07, 2017, 07:10:40 PM »
Quote
Laff Ben....  I may come back over there, but it would be to bring another set of rifles.  These having 9 numbers apart that would index 90 degrees from 12 o'clock.  In fact, the last test one barrel was 180 past index and the other was 180 pre-index, so a new test having 90 degree difference should land one at 3 o'clock and the other at 9 o'clock. 

I see someones trying to make a few bucks selling to the local 'hood.  "See when you turn this gun sideways the sights are right there!  I have them for leftys and rightys, how many do you need?" rofl

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #77 on: December 08, 2017, 03:40:39 PM »
Following GMs lead, I took a few pics.   wink1

I have discussed these cuts extensively with RM and it is of our opinion that the following list of guns have had their extractor cut PRIOR to being mated to to receiver.

Russian
Chinese
Romanian
Albanian
Yugo LBs and M59s
East German

It is of our opinion that the following list of guns have had their extractor cut AFTER being mated to to receiver.

Yugo 59/66
North Korean



The evidence.  Click to enlarge.







































« Last Edit: December 08, 2017, 03:51:31 PM by Loose}{Cannon »
      
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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #78 on: December 08, 2017, 03:51:34 PM »
Great array of pictures clearly showing the differences! I concur, the M59/66 and NK stand out as having extractor groove cuts done post assembly...cuz I knew all of you were just waiting for my expert opinion  rofl

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Re: SKS Barrel/Receiver Index Numbers Identified and Verified
« Reply #79 on: December 08, 2017, 04:12:11 PM »
The East German is messing with the rest, right now.

Extractor cut is below receiver ledge.

I have it apart right now, scouring the receiver for any clues as to how they marked it.  So far, no good info.

But I'll keep looking.