bore size

alan self

January 12, 2009 18:59

I've seen there is people talking about reloading with .308 bullets in 7.62x39 brass in a .310/311 bore. I'm just a newbe but that just don't seem right. I know its only .02/.03 but what happends if you start using in say a barrel with .315 bore.
Is it ok to use cast bullets w/gas checks in ak's 30-30 etc. I know there is a lot to learn but I'm almost getting to old to learn. In my younger years my dad didn't want me playing with guns now I got a lot of catching up to do. alan

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  nononsense

  January 13, 2009 06:19

  alan self,
  
  I'm not going to get too deeply involved an answer here because I and several others could write books about subjects such as this. I will absolutely suggest getting a book from the library or buying a copy of Lyman "Cast Bullet Handbook: 3rd Edition". This is well written and chocked full of great information about cast bullets.
  
  Another good reference book is Cartridges of the World.
  
  Shooting jacketed under size bullets in an oversize barrel results in plasma blow-by which will erode the base and sides of the bullet causing some inaccuracy. In the case of lead bullets and under size, the blow-by will result in additional leading and fouling. The gas checks help protect the base but do nothing for the sides of the bullet. In both situations the bullet contact with the rifling is minimal and the bullet can skid and slip in the bore resulting in inaccuracy.
  
  Slug your barrel the get the accurate measurement so you can select the correct bullet.
  
  Best.

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  Rocky Raab

  January 13, 2009 07:19

  It's a complicated issue that can't be completely addressed here. As mentioned, it would take a book to do it.
  
  Jacketed bullets work a bit better if undersized compared to lead bullets. They're simply better protected.
  
  All bullets "bump up" a bit when they are accelerated by the powder gas. So by the time it gets fully into the rifling, a bullet might have obturated (that's the correct term) up to proper diameter. Naturally, there is a limit to how far it can obturate, so a bullet that's a little small might work fine, but not if it's grossly undersize. The composition of the jacket and core, its hardness and its friction coefficient all have an effect here.
  
  Lead bullets obturate easier because they are softer. But they also tend to skid or smear more when they hit the rifling - for the same reason. That's why it is almost always better if a lead bullet is a bit LARGER than the bore. Combined with the tendency to fatten anyway, a tight lead bullet will seal even tighter, strip less in the rifling, and spin up with less shearing.
  
  I don't shoot any semi-auto, so I can't address an issue specific to an AK/SKS, except to note that many gas systems can be ruined by getting lead in the works. That's why cast bullets are forbidden in some actions.

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  alan self

  January 13, 2009 08:54

  Thanks. True to a lot of things There just not an easy answer. But its a little clearer now.

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  jonk

  January 13, 2009 11:27

  I have an SKS that measures .310. Shoots .308 M2 surplus bullets as well as anything, but then it's an SKS.
  
  For real accuracy I wouldn't do it, but it won't hurt anything.
  
  Now with cast lead, it can lead the snot out of the bore really fast if undersized.

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  cchance

  January 16, 2009 03:49

  When considering a bullet size, isn't the groove diameter more pertinent than the bore? The OP is discussing the bullet diameter in reference to the bore.

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  iwannausername

  January 16, 2009 03:53

  On top of that, some guns in 7.62x39 use a .308 some use the .311. Ruger's Mini-30 (at least when it first became available, not sure of now) actually uses a tapered bore, 311 at the chamber end and 308 at the muzzle.

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  nononsense

  January 16, 2009 12:31

  cchance,
  
  Welcome to the Gunbroker Forums!
  
  "When considering a bullet size, isn't the groove diameter more pertinent than the bore? The OP is discussing the bullet diameter in reference to the bore."
  
  In a perfect world, everyone would know all the correct, descriptive terms to use. That's why we encourage folks to read for context and content.
  
  In this instance, the term 'bore' is the generic reference to the inside diameter of the barrel so that we don't have to be specific as to which diameter we're writing about. When specifically referencing the design of the inside of the barrel, we use the terms 'land diameter' and 'groove diameter'.
  
  Best.

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