What causes small nodes??

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jsthntn247, Apr 21, 2015.

  1. jsthntn247

    jsthntn247 Well-Known Member

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    I've been working up loads for a new 243. It's a 7 twist 24.75" Criterion. The barrel has a straight taper from 1.2 to .760 at the muzzle and weighs 4.5lbs. I've worked up loads with 115 dtacs with H4350, H4831, and H-1000. The nodes seem to be small with this barrel with all powders, .2-.4 at most. I've had barrels with 1-1.5gr nodes where the poi on both elevation and vertical does not shift for several powder charges when doing the OCW load test method. What causes small nodes??
     
  2. FearNoWind

    FearNoWind Well-Known Member

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    Perhaps it's' a wild theory; but:
    I am neither a physicist nor ballistician, but I have some experience that leads me to suspect that there may be "false" nodes created (aggravated in barrels with less mass) in rifle barrels. In addition to the harmonic vibrations along the length of the barrel, a certain amount of expansion occurs when pressure developes in the chamber, producing circular expansion along the bore. The ratio of circular expansion coupled with the frequency of the longitudinal harmonic vibrations along the length of the barrel combine to produce the nodes. If the circular expansion is constant and relatively small, as it might be expected to be in a heavy straight (not tapered) barrel, it should have little affect the longitudinal harmonic vibrations. But with a light weight tapered barrel the null points of the longitudinal harmonic vibrations will change, modulating the frequency of the longitudinal vibrations.
    When I get the type of results you describe I create sample loads within the range that produce the best results and move out to 300 yards. One will usually distinguish itself as better than the rest.
     
  3. Canadian Bushman

    Canadian Bushman Well-Known Member

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    I think its a combination of the cartridge componets and barrel.

    Have you shot any other bullets through that barrel?
     
  4. jsthntn247

    jsthntn247 Well-Known Member

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    I wanted to shoot these or 105 Hybrids. Im going to shoot a tac match with it this weekend and see how the vertical holds. 42.0 and 42.2 gr of H4350 were both under .25 moa with the same poi, however 41.8 and 42.4 opened up and poi shifted and 42 was on the hot side as well. I settled on 47.6gr of H-1000 for now as H1000 seemed to have a wider node from 47.2-47.8 where it stayed on the same vertical line on the target but shifted horizonatally from left to right with each .2gr increase. Just can't seem to get anything to stay consistent which leads me to think it will shoot great one day and 1moa the next if the temp or altitude changes.
     
  5. Canadian Bushman

    Canadian Bushman Well-Known Member

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    I was gonna suggest the 105 and h4350 but apparently you were on top of that one.

    Not too sure why your rifle could be so finicky. If you make some headway on it id be interested to see what you discover. Ive had the same issue just not to the degree of what your experiencing. Usually a different bullet or powder helps me out of the situation.
     
  6. Mikecr

    Mikecr Well-Known Member

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    Another theory; Short stiffer barrels produce narrower nodes than long flexible barrels.
    It's very easy to tell when you're in/out of nodes with longer barrels. Its clearly good or really bad.
    Short barrels seem more forgiving because while it's easier to slip out of nodes, the muzzle is still moving less than the longer barrel.
    With this, it's easier to identify 'best' load with a longer barrel.
     
  7. Canadian Bushman

    Canadian Bushman Well-Known Member

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    I wouldnt debate that.
    My Straight tapers have smaller nodes then my palma contours.
     
  8. jskmtd

    jskmtd Well-Known Member

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    Genetics :)