Bullet Jump

Discussion in 'Reloading Forum (All Calibers)' started by richraceri, Nov 16, 2018.

  1. richraceri

    richraceri

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    I was wondering why some rifles seem to be much more accurate after minimizing the bullets distance to the rifling and others do very well at magazine length and big jumps? I imagined as the powder burns the brass expands from back to front assuring that every bullet leaving for a fractional instant is always concentric. Meaning in the first fractional second the back .010" of the bullet neck is fully and tightly expanded to the necked chamber centering every bullet "before it even leaves" to whatever neck chamber the machinest made. In short every bullet is aimed directly at the rifling by the earliest part of the gas expansion. Of course if the brass is thicker on one side this violates the perfect alignment of every bullet with the rifling even before it leaves the brass theory, but good brass is pretty even walled. So assuming your brass is decent or turned (for thickness variations, not diameter or concentricity) does it all come down to every bullet jumps either aligned with the rifling if the machinest got the original neck / bore alignment concentricity within .00001 or takes off at a built in angle because he machined it .0001 off? Then I thought, any amount of off-set in original machining concentricity is an angle which is multiplied by the distance of the jump. Having run a lathe there are lots of tricks a great CNC machine or machinest can do to make it more accurate but it will never be perfect if you go enough decimal places. Repeating myself cause I like too, pretend its a conclusion: Does it all boil down to how good the original machining was that determines if your particular rifle is a don't much care jumper or a gotta have jammer? Is the rifle's tolerance for jump a direct indicator of machining accuracy even if we can't measure .00001 (add whatever 0s you like or point out I used too many for dramatic effect) concenticity? Is a good jumper a better barrel at heart and the machinest you want to buy beers for? Then, even if you jam a miss-aligned barrel does it completely solve the problem or does the bullet still feel the angle via loading when it leaves the brass and carry forward a 10% vibration even though you cured 90% by jamming? Newbie to forum, probably should have started with something like whats your favorite color brass, Sorry, had fun thinking about the why today. PS: Of course a great rifling "lead in" helps all ills, but probably doesn't change the original question.
     
  2. rammac

    rammac

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    I haven't got a clue but I do know that you're wrong about the brass expanding from back to front.
     
  3. abersfelderami

    abersfelderami

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    ...and your original question was?:)
     
  4. Fotheringill

    Fotheringill

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    Welcome to the forum.

    Each barrel and each chamber is different. Getting everything to less than 1/10000 is not going to happen. Take solace in the thought that however finicky a centerfire may be, the finickness is ten times worse with a rimfire.

    If you are not shooting at extreme distances, you are not going to have any issues in the results at the tolerances mentioned.

    Brass may, indeed, flow in any direction it wishes to do so; towards the neck, away from the neck, etc.

    As to jump or jam, my original Savage 12 in .223 liked a jump of 80 thousandths. It got shot out and the new Shilen likes a jump of .002 best. There is no set formula, but try the Berger Bullets method at its website.
     
  5. cw308

    cw308

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    Well not all rifles shoot better into or close to the rifling . My 308 benchrest rifle started out as a Remington 700 LTR , the distance to the rifling was so long the bullet almost left the case before it came close to the lands . So I went with the listed AOL 2.800 and worked up a accurate load . After 4000+ through the barrel I changed the barrel , blueprinted it completely an bedded .The leade in the chamber is now very short . I now had two jobs to do best jump or jam and best powder charge . Your going down that rabbit hole for sure , thinking of every possible thing that can improve accuracy . Most is trial and error . When you think of making reloads in . 001 let's go back to shooting and aiming in .001 it's not all in the ammo.
     
  6. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    I'd say that more often than not, seating depth preference lies in the design of the bullet, not the rifle. It's not at all uncommon to find a given bullet's seating depth optimum range to be similar across a wide range of different rifles.
     
  7. Steve Ladino

    Steve Ladino

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    Lots of good theory here but surely there is a lot more. Sierra Bullets must consider this phenomenon resulting in their LT line of bullets often fired from rifles with magazines holding ammo with seating depths far from the lands and they shoot well, ie. AR's and Weatherby Mag chambers.
     
  8. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    That’s a difficult question that nobody I know really knows the answer to. In my opinion, keeping the bullet straight is the most important part of keeping a bullet shooting to its potential (which isn’t the same for every bullet).

    How seating depth impacts precision is somewhat of a mystery. It is in my experience cyclical. I have shot bullets that shoot well close to the lands AND with very large jumps (over .120”). I’m not sure why. Could be some sort of oscillation on bullet release. Could be something to do with engraving force. Maybe something else entirely.

    I do not believe machining tolerance impacts seating depth sensitivity in practical terms. However, I would not be surprised to see very tight freebore diameter reduce sesnstivity to seating depth. And it’s possible that neck turning decreases that sensitivity, but I have no good evidence of that. Perhaps the bechrest guys can chime in on either of those issues. Theyve tried just about every possible configuration on the 6PPC.
     
  9. Meangreen

    Meangreen

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    This...along with lead angle, is my theory.
     
  10. richraceri

    richraceri

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    Thanks ideas, after more thought like with any complex condition (a lot of variables) it’s usually wrong to look for a single solution and say that’s the reason. My experience is it’s 10% that, 15% that etc. it was just fun to wonder if the back of the neck expands first centering the bullet before or as it leaves the brass. In racing never sell a great carburetor, in shooting a great barrel. Happy turkey day.
     
    Jennb likes this.
  11. Bart B.

    Bart B.

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    The body brass behind the shoulder where body brass is typically thinnest.
    Not quite, note the case neck wall is thinner than the body wall. It starts expanding when the body wall does

    Bottleneck cases headspacing on their shoulders center their shoulder in the chamber shoulder before the primers fire. Rarely does case necks center bullets to the bore. Straight case necks centered on case shoulders do that. Usually, no such cartridge part forward of its shoulder touches anything when the primer fires; bullets seated to engage the throat excepted.

    A 30 caliber bullet that needs 10 pounds of force to push it forward in the case neck requires only 135 psi in the case to move it. The bullet is starting into the rifling at low pressures that don't expand case necks against chamber necks. A couple thousand psi in the case will push the bullet full into the rifling.

    A 22 caliber bullet needing a force of 5 pounds to move in the case neck needs 129 psi in the case.

    No part of cases expand any significant amount before bullets move.
     
    Last edited: Nov 21, 2018
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  12. Bart B.

    Bart B.

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    Lots of cases have their shoulders set back up to several thousandths from firing pin impact before the primers fire. As pressure builds, the body behind the shoulder expands and grips the chamber at that point. Meanwhile, the back of the case stretches back. Reduced loads more than 10 to 15 percent below maximum may not have enough pressure to press the head against the bolt face.

    Cases so loaded will have less headspace after firing than before. Primers are pushed out past the case head 'cause pressure wasn't enough to reseat them as the case head doesn't stop against the bolt face.
     
    Last edited: Nov 21, 2018
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