Author Topic: SKS Barrel/Receiver Index Numbers Identified and Verified  (Read 26090 times)

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Offline Loose}{Cannon

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SKS Barrel/Receiver Index Numbers Identified and Verified
« on: November 29, 2017, 05:57:46 PM »
In a recent brain-storming thread located here, the age old topic came up regarding the large corresponding numbers found stamped into the barrel lug and receiver on threaded barrel actions (minus Yugos).  Most people over the years have said these numbers were applied to keep the barrel and receiver together when separated.  I for one have never bought into this notion and have been convinced these numbers are some type of numbering/measuring system vital for mating barrels to receivers at the factory while ensuring perfect rotational indexing during the assembly process.  Admin GM and RM actually had the light bulb go off at the same time without collaboration as to how these numbers may have been used. 

We think that the two numbers on the barrel and receiver that you tend to see on every Russian, Romanian, Albanian, and threaded Chinese SKS are in fact rough indexing numbers that the builders used when mass producing those particular components.  From all the data RM has compiled, the numbers run from 0 to 35 for a total of 36 different numbers.  As there are 360° in a circle, it would seem that each number indicates ~10° variance in the start of the the thread for ease of indexing the two parts together w/o a lot of trial and error. 

If we are right on this, then for example a receiver with a #10 would mate to a #10 and index the barrel within 10° prior to torquing.  If, however you took and replaced the correct barrel with a #19, I would expect the indexing of the two parts to be approximately 90° off give or take ~10°.  A #28 barrel would be 180° off and so forth. 




For reference here is a quote from myself in the brain-storming thread. 


Quote
Its nearly impossible to clock the starting position of male or female threads even on the most modern CNC machines yet alone in the 50s.

What your looking at is rotational degrees that are plotted prior to mating of the components.  You would mark the witness mark 90° from 12 o'clock and thread the barrel into a jig. This jig would have 360° marks all the way around it in 10° increments from 0 to 35.  When you seat the barrel all the way into the threaded jig your witness mark will land on a number.  You stamp that number which literally plots the beginning location of the first thread.  You do the same on the receiver using a male jig and stamp the number. 

You now have receivers and barrels on standby numbered 0 through 35 representing the rotational degree of the begging thread.  You achieve perfect index every time you match the numbers. 

It explains the witness marks, the numbers, and the entire process of how they indexed barrels to receivers.




So, how can we prove this theory to be fact? 

What we propose is that we take two guns from the same country of origin that said numbers are differing by 18 digits which would represent a 180° difference between the two, take the barrels off the receivers, swap both barrels to the apposite receivers, and if we are correct.... We should have two barrels inverted 180° from where they should be when seated.  So in short, FSBs and gas-blocks will be facing downward at the 6 o'clock position and the stock ferrule facing upward at the 12 o'clock position.

To put this plan into action, we a have recruited none other then SKSFiles long time member and gunsmith Ben Murray of Murray's Gunsmithing to perform the test.

Below, I present to you the test subjects. One is a 1958 Chinese /26\ marked as a 13, and a 1959/60 letter H series Chinese /26\ marked as a 31.   These two numbers were specifically chosen by me from my collection because 31-13=18 and this 'should' represent a difference of 180°.





And just to have a little fun with this, I created this thread as a poll so you can vote whether or not you think our theory holds any water.   :)     Poll closed with 14 yays, and 0 nays.   


Remember, we could very well be completely wrong and have to go back to the drawing board, but atleast we here at SKSFiles are giving it a try.   wink1

Results will be posted in two days on the evening of Friday 12/01/17
« Last Edit: December 02, 2017, 10:19:40 AM by Loose}{Cannon »
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Offline Loose}{Cannon

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Re: Barrel Index Numbers?
« Reply #1 on: November 29, 2017, 05:59:37 PM »
Ladies and gentlemen, members of SKSFiles.....   We have 180° inverted barrels.

 dance2 dance3 locomotive1 ----------------  declare1

For your viewing pleasure I first present to you barrel #31 mounted and torqued on receiver #13.  Since the RSBs were loose, we simple rotated them to about where they should align with the rest of the barrel components.  The important thing to note is the 180° inverted stock ferrule, gas block, front sight block, and of course the witness marks.   wink1
















Now for barrel #13 on receiver #31 we didnt torque the barrel to the receiver. We have ZERO doubt it would have also indexed at exactly 180° also, but we did this to roughly show where mating of the two surfaces began prior to torquing the remainder of the way. BUT, as you can see in the first picture, we hand tightened this barrel and the threads on this one were a bit tight and we didnt get it quite all the way seated by hand.  Absolutely no doubt this barrel would have indexed exactly 180° inverted like the other.   thumb1











It is in my opinion these rotational indexing reference numbers would have allowed them to achieve within 20° to 30° PRIOR to index and that amount would be torqued until the witness marks align.  When we reinstalled the proper barrels on their respective receivers this is exactly what was observed.   wink1


And dont fret....  the rifles are back to normal without a single adverse affect.  clap1










« Last Edit: December 01, 2017, 05:34:10 PM by Loose}{Cannon »
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Online Boris Badinov

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Re: Barrel/Receiver Index Numbers?
« Reply #2 on: November 29, 2017, 06:24:38 PM »
Can you add an extra choice... a hedge of sorts?

"Probably correct"

Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #3 on: November 29, 2017, 06:29:29 PM »
Sorry Boris.... no 50 shades of grey here.  :))
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Offline martin08

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Re: Barrel/Receiver Index Numbers?
« Reply #4 on: November 29, 2017, 07:12:56 PM »
Having replaced two crapped out Yugo barrels with chrome lined Chinese barrels, I can say that witness marks are not a homogeneous thing.  The mating parts do have to index properly for the extractor cut to line up correctly, and my two projects required removal of material (maybe a couple thousandths of an inch?) before the barrel indexed properly in the receiver.

I think the experiment will yield exactly the results which are predicted.  Great idea to have an independent gunsmith to do the work, too.

Offline John Galt

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Re: Barrel/Receiver Index Numbers?
« Reply #5 on: November 29, 2017, 07:17:02 PM »
This will be interesting!   popcorn1
When the Cambrian measures were forming, They promised perpetual peace.
They swore, if we gave them our weapons, that the wars of the tribes would cease.
But when we disarmed They sold us and delivered us bound to our foe,
And the Gods of the Copybook Headings said: "Stick to the Devil you know."
Rudyard Kipling  1919

Offline MxwllBkr

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Re: Barrel/Receiver Index Numbers?
« Reply #6 on: November 29, 2017, 08:07:33 PM »
 
   popcorn1
X2

I like it! And errrrrrr ummmmm glad your sacrificing yer own  rofl

Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #7 on: November 29, 2017, 08:10:04 PM »
   popcorn1
X2

I like it! And errrrrrr ummmmm glad your sacrificing yer own  rofl

Its uh..... removable part?   Geez.....  it gonna ruin or something?

This isnt any different then unthreading a bayo screw...... on banians no less.
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Offline newchi

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Re: Barrel/Receiver Index Numbers?
« Reply #8 on: November 29, 2017, 08:14:44 PM »
Quote

Its uh..... removable part?   Geez.....  it gonna ruin or something?



Probably not but the sight will be on the bootom and the bayo on top for evermore!!!
(unless you change them back again)

Offline martin08

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Re: Barrel/Receiver Index Numbers?
« Reply #9 on: November 29, 2017, 08:15:26 PM »
With care taking place with both of your carbines, you likely won't even be able to tell that they've been apart when all is done.    The mating parts should return exactly as original.

But is is a nice gesture that you volunteered a couple of guinea pigs for the experiment.

Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #10 on: November 29, 2017, 08:18:32 PM »
Murray has the right stuffs.   :P

Besides....  Ill be hovering over his shoulder.    chuckles1
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Offline Greasemonkey

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Re: Barrel/Receiver Index Numbers?
« Reply #11 on: November 29, 2017, 09:28:37 PM »
Ain't no sense in hiding my choice..I'm secure with a big.. YUP!!  thumb1

And... I figure if uber nerd Professor RM also came up with the same conclusion.. Some surious stuffs is about to happen..


If'en I'm wrong.. whoopie  fart1 I'm used to it, I have been married and wrong for 23 years.. just ask the wife. Ok..fine back to the drawing board.


Quote
but at least we here at SKSFiles are giving it a try.   wink1

Love it....getting the bobblehead bible out and thumping it, preaching this is fact..  nea1  Proof is fact!! not guessing, not assuming, not bible thumping..

no grey area..

Simply... it is or it isn't.... to be determined 12/01/17  :o thumb1

ya done yet..is it the 1st yet... did it work?  senil1
I'm going to make him an offer he can't refuse......

Leave the gun, take the cannoli.

I said I was an addict........I didn't say I had a problem

Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #12 on: November 29, 2017, 10:08:59 PM »
You aint right!      rofl
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Online Boris Badinov

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Re: Barrel/Receiver Index Numbers?
« Reply #13 on: November 29, 2017, 11:15:00 PM »
I'm with GM...tell Murray to " step on it". I'm getting old waiting popcorn1.... geezer1

Offline newchi

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Re: Barrel/Receiver Index Numbers?
« Reply #14 on: November 29, 2017, 11:34:17 PM »
Quote






Just as a nerdy FYI, to find the opposite of any of these 36 numbers the old +2/-2 trick works great.
Either +2 to the first number and -2 from the second or the other way as required.
31 = 3 -2 is 1 and 1 +2 is 3 - 13

Offline Phosphorus32

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Re: Barrel/Receiver Index Numbers?
« Reply #15 on: November 30, 2017, 02:40:19 AM »
The empirical approach  clap1  thumb1 Eagerly awaiting the results.

Offline firstchoice

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Re: Barrel/Receiver Index Numbers?
« Reply #16 on: November 30, 2017, 09:12:43 AM »
Voting "Yes". Agree that that was the correct theory. Now, you're taking the step of actually proving, or disproving the common theory. Too bad more samples couldn't be used. But two's a good start. Looking forward to the results.

Were these indexing marks used on Mausers and other mass-produced milled, threaded receiver rifles?

firstchoice

Offline Ikecornell

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Re: Barrel/Receiver Index Numbers?
« Reply #17 on: November 30, 2017, 06:06:40 PM »
Is a (31) considered standard, my letter gun is a (31) also.
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Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #18 on: November 30, 2017, 06:14:54 PM »
Is a (31) considered standard, my letter gun is a (31) also.

0 to 35 is whats marked on any given gun.  35 is the highest observed.
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.

Offline Loose}{Cannon

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Re: Barrel/Receiver Index Numbers?
« Reply #19 on: November 30, 2017, 06:15:55 PM »
Less then 24 hrs and we will know one way or the other.   :o
      
1776 will commence again if you try to take our firearms... It doesn't matter how many Lenins you get out on the street begging for them to be taken.