Unstable bullets.

Discussion in 'P-3AT' started by edd_browne, Jan 29, 2009.

  1. edd_browne

    edd_browne New Member

    65
    Nov 23, 2008
    I have been discovering that many handguns (and some rifles) fire bullets
    having excessive wobble/yaw, and even reversal, before hitting targets.

    I wonder who has had elongated keyhole hits with a p3at,
    at what distance, and using what ammo.

    Some tentative factors in yaw/wobble seem to be ...
    --Short pre-fire engagement of bullet in rifling.
    --Loose fit in rifling from wear or bullet size.
    --Blunt or short bullets.
    --Supersonic and transonic speeds.
    --Barrel twist too large or too small.
    --Buildup of crud on rifling after 100 rounds or more.
    --Short barrels, particularly Seecamp.
     

  2. edd_browne

    edd_browne New Member

    65
    Nov 23, 2008
    That appears to work well for rifle bullet numbers.
    When I plug in .38 acp  numbers, the stability factor
    is in area of 50, far beyond super-stable.

    There must be a spreadsheet for pistol somewhere.
    But gun characteristics are crucial, such as
    barrel length, so actual range results are the key.

    Thanks.
     
  3. k9pound

    k9pound New Member

    30
    Jan 18, 2009
    I have not had this problem with my P-3AT, but I do know that this can occur with short barreled weapons and smaller diameter bullets. Worn rifling in weapons built in the last 5-10 years is less likely due to better materials and better machining
     
  4. sniper7369

    sniper7369 New Member

    145
    Aug 25, 2008
    Williston, FL
    The only time I've ever had this problem is with my P-11 shooting a 147gr hard cast flat point. Every bullet went through the target sideways. I my M&P 9c, 3904, and SUB2K they shot fine.
     
  5. doubloon

    doubloon New Member

    Jan 5, 2008
    Houston-ish, TX
    Somehow I thought bullet stability was more a factor of the dimensions and weight of the bullet calculated against RPM and the desired effective range.

    In other words, if you know the velocity and the twist rate the RPM is a given.

    I would think the distance at which the bullet is expected to perform matters much more in a stability calculation than the barrel length since bullets inherently become more unstable the farther they travel. A bullet that is stable at 100 yards with a specific weight, dimension and spin might be the aerodynamic equivalent of a charcoal briquette at 300 yards unless the RPM is boosted.

    I know shorter, heavier bullets are easier to stabilize at lower RPMs than longer lighter bullets ... for a given range. It's fairly easy to make anything stable, even a cinder block, out to 20 feet.

    But maybe the formula from the original link is taking distance into account somehow and maybe the 20.62 in the formula below, from the spreadsheet, is some crude fudge for effective range.
    =((B13*(20.62*(B15^2.25))^2)/((B14*(B12^2.25))^2))/(1-(B11/5705))

    There's another calculator here with a slightly different formula.
    http://accurateshooter.net/Blog/millerformula.xls
    =(30*C5)/((C7/C4)^2*C4^3*C6/C4*(1+(C6/C4)^2))*(C8/2800)^(1/3)*((C9+460)/(59+460)*29.92/C10)

    Of course anything based on a calculation assumes a clean, straight bore with good rifling as well as a symmetrical projectile to produce an accurate estimation since lead fouling, bad lands and crooked barrels or bullets would make anything based on a calculation pointless. If the bullet is flat on one side it's going to destabilize immediately.

    But, I'm more or less guessing based on a rudimentary knowledge of physics. If someone could post an absolute formula for calculating bullet stability that would eliminate the guess work and the need for a poll.