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.
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.
Results will be posted in two days on the evening of Friday 12/01/17