SKS barrel: factory tolerance for bore diameter

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dron

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Well, we all know by definition that the bore of a "new" SKS should have 7.62 mm (0.3") in diameter (between lands). But how often you can see this magic number in reality, even on so-called "un-issued" rifles?

I've checked many, and only few had this 7.62 mm nominal diameter (measured at a muzzle). All others were in a range between 7.64-7.66 mm and up, including some obviously "unfired" 1955/56 K-letter beauty I saw recently. Is it normal at all to have such variations on "un-fired, un-issued" rifles?

I assume, there is a factory tolerance on the bore diameter. But which one? Does anyone know?

FYI, according to Russian military standard for SKS rifles, a cutoff diameter, which renders the rifle non-serviceable, is 7.772 mm as long as the rifle meets accepted shooting accuracy.
 
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I don't underhand what your trying to glean from this. While a tolerance of +.0015" is a rather poor tolerance for a commercial barrel, it is more than acceptable to a rear-lines quasi battle-rifle. While suh a large tolerance is unacceptable in a match grade barrel (bore and grooves usually held to within .0001", or better yet, perfect nominal at the breeh and evenly tapered to nominal -.0001 at the muzzle) it will have little to no effect on a military gun. Also, bore diameter is far less relevant than groove diamter. You might be able to get a better analysis of groove diameter, and its effects on accuracy and velocity than you could by analyzing bore diameter alone.
 
Blastattack said:
I don't underhand what your trying to glean from this. While a tolerance of +.0015" is a rather poor tolerance for a commercial barrel, it is more than acceptable to a rear-lines quasi battle-rifle. While suh a large tolerance is unacceptable in a match grade barrel (bore and grooves usually held to within .0001", or better yet, perfect nominal at the breeh and evenly tapered to nominal -.0001 at the muzzle) it will have little to no effect on a military gun. Also, bore diameter is far less relevant than groove diamter. You might be able to get a better analysis of groove diameter, and its effects on accuracy and velocity than you could by analyzing bore diameter alone.
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Well, it's quite simple. In fact, in my OP I had two questions:

1. traditionally in Russia a bore diameter is measured between lands. So that's why 7.62 mm is in a technical documentation for a rifle. After production each rifle was inspected by QA team to meet certain specifications, including the bore diameter. So my first question is then this: what was the tolerable range for bore diameters to pass such an inspection? 7.62-7.6(?). Why this question? well, I have otherwise looking pristine 55/56 rifle with a bore diameter 7.64 mm. I wonder why not a nominal 7.62mm.

2. During the service (either shooting, or abuse while cleaning), lands will be wearing out in the first place, gradually increasing the bore diameter to the point, the rifle becomes un-serviceable and thereafter commissioned. This cutoff diameter is set to be 7.772 mm as per the official SKS repair manual. So, here is my second question: could the bore diameter (between lands) be an indicator of extensive use of the rifle? Say, if you have 7.62 mm - 7.63 mm, then you are lucky owner of a truly unfired rifle, if 7.64 mm - 7.65 mm - you got a nice shooter, if larger than 7.65 mm - well, you have a noise maker for bear distraction.
 
For starters, I believe that all European rifles are measured between the grooves and North American are measured between the lands, or at least this seems to ring a bell in the back of my brain from class still.... So how are you measuring your barrel. If you are not pressing through a lead slug and then measuring from the lead, your measurements are meaningless anyhow. At least that is how every book I have looked at tells you how to measure a bore accurately.
 
Porschemann said:
For starters, I believe that all European rifles are measured between the grooves and North American are measured between the lands, or at least this seems to ring a bell in the back of my brain from class still.... So how are you measuring your barrel. If you are not pressing through a lead slug and then measuring from the lead, your measurements are meaningless anyhow. At least that is how every book I have looked at tells you how to measure a bore accurately.
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A 30 cal barrel such as used on a 308 or 300wm is 307-308 between the grooves. 298-300 between the lands.
 
ciphery said:
Well I have a never isued Polish M44 with a .3155" bore that's +.0045" so having a +.0015" tolerance is no big deal.
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.3155-.0045=.31" in metric is 7.874 mm. Which diameter is it, between lands or grooves? It is neither one thing nor the other. For grooves (nominal 7.92 mm) is too small, for lands (nominal 7.62 mm) is way too big (unless this rifle had seen a lot of battles).
 
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