Barrel Bore Curvature

Discussion in 'Gunsmithing' started by Edd, Feb 1, 2012.

  1. Edd

    Edd Well-Known Member

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    How many of you index the curvature of the barrel bore and how do you determine the direction of the curve?
     
  2. grit

    grit Well-Known Member

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    Dial in the section of barrel where the chamber will be. Then indicate the muzzle end, and mark the high spot.
     

  3. Edd

    Edd Well-Known Member

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    This topic hasn't got much action. Is it because everyone does it like Grit or is it because not many even index the bores?

    Using search, I've found more reading about indexing flutes than I have about bore curvature.
     
  4. justgoto

    justgoto Well-Known Member

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    I never heard of it. I subscribed to the thread in the hopes of a discussion on the condition/issue.
     
  5. grit

    grit Well-Known Member

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    I doubt many besides smiths ever think about it. I don't believe factory smiths pay it any attention at all.
     
  6. Coyboy

    Coyboy Well-Known Member

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    Your question is simple but the answer is much more complicated than "who does what"

    "How do you find the curvature?" you look down the barrel either in or out of the lathe.
    Your eye needs to be a little trained to do this.

    Don't expect to either see a curve, or have a curve that only goes in one direction for the length of the barrel. Every manf. seems to have a different handle on how to create a straight bore, and I'm sure some don't care. I typically use the barrel makers who try and drill nice straight holes.

    Now depending on how the smith sets-up, and chambers, the barrrel will determine how effective or needed it is to indicate the curve. Or weather in the end it makes any or no difference at all.

    If you ream start to finish with the reamer you may chose one way, if you drill and bore before reaming you may do it the other. Their is a multitude of ways to indicate and cut a chamber. I happen to think my way is the best, thats why I do it that way. Ask any smith and thats likly what he will tell you.

    In the end no matter which way it was done, if it was done properly the difference in the guns ability to shoot will likly be more effected by other things in the process of the assembly, components and implementation.
     
  7. SidecarFlip

    SidecarFlip Well-Known Member

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    Interesting subject.

    Every (considered straight bore or line) is curved if the tolerance of measurement is fine (close enough). Gaging in thousands is like a potholed road compared to millionths..... or.... 0.001 versus 0.000001.... and it has everything to do accuracy of the machine tool, tooling, ambient temperature at which the machining took place, remperature at the cutting edge, operator skill and a multitude of other varibles.

    In reality, nothing is straight, everything has curvature to some degree when the unit of toerance become acute.

    So, the term straight without curvature applies to the degree of acceptable accuracy in the machining process, the cost of the process and the degree of acceptance by the end user in relationship to the cost......
     
  8. Edd

    Edd Well-Known Member

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    Knowing the amount of curvature would be important in knowing the effect of it. I considered including that in my question but I thought that might be asking too much. But if anyone who finds the direction of the curve, when installing a barrel, can determine the amounts they are finding, I'd be interested in knowing the number.
     
  9. sp6x6

    sp6x6 Well-Known Member

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    Now I know why my gun shoots left:D
     
  10. Coyboy

    Coyboy Well-Known Member

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    Edd there is no real way to measure the amount of curve accurately unless you cut the barrel in half and measure the opposing wall thicknesses. This would be just a little costly. I guess one could indicate both ends true, than a 30 inch grizzley rod w/bushing, held by the tail stock, rod run 15" into the barrel. Then indicate on the rod, right next to the breach, and take that measurement x2 would equal runout.

    The effect? I guess you need to clarify the effect on what?

    Based on what I have seen with barrels going thru the shop is that the curve is usually very minimal, if I had to guess I would say in the center of the barrel, with both ends indicated, there may be about .010-.020" of TIR. I have seen some bad enough that it may be .035", If you ever got one with .050" I would say it was a rare and extreme case.
     
  11. Edd

    Edd Well-Known Member

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    The effect it has on the direction the bullet goes.
     
  12. Coyboy

    Coyboy Well-Known Member

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    Take the amount of "ramp" in the barrel per foot times the distance to target in feet would equal the error from theoretical straight line. assuming the barrel has a one direction curve indexed up. So from centerline of barrel if it curves up .010" we could calculate a 3" point of impact error from what a perfectly straight barrel would produce, Theoretically.

    Depending on weather the bore was indicated in one of two styles;
    Straight in the first 3" of breach and let the muzzle run out wildly with the curve,

    Or indicate in the throat and muzzle dead nuts, this would lesson the effect by half, theoretically.
     
  13. Edd

    Edd Well-Known Member

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    3 inches at what distance?
     
  14. SidecarFlip

    SidecarFlip Well-Known Member

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    I'm with you in shooting left.