ballistic calculator accuracy

Discussion in 'Long Range Hunting & Shooting' started by patsygarret, Nov 5, 2010.

  1. patsygarret

    patsygarret Member

    Messages:
    20
    Joined:
    Jan 25, 2010
    Guys I was wondering how accurate you have found jbm trajectory simplified calculator to be. The reason I ask is I was in the process of getting my load doped and with a 100 yard zero I found the 500 yard come up to be right at 9 inches too high. The groups were right at 3 inches the load is a .338 lapua 300 smk at a chronied 2685 av. Rifle is a stock remmy w 5.5x20 nf. My previous longish range shooting was typically done w the ballistic recticle scopes., so the dialing in is new for me. Any help is appreciated.
     
  2. Ridge Runner

    Ridge Runner Well-Known Member

    Messages:
    987
    Joined:
    Dec 13, 2002
    its only as accurate as the data its fed.
    most probably your actual BC is different from the published one.
    Here is what I do, don't know if its right or not but it works well for me.
    sight in at 300 yards, on a large target
    shoot (with a 300 yard zero) at the same target at 400, 500, 600 yards
    measure your actual drops
    tweak your BC in the ballistic program until your chart matches your actual trajectory.
    In my 7mm AM I must use a BC of .62 to hit where I aim with a 160 accubond, nosler says its .531
    I use a .7 BC for a 140 berger, berger says its .64.
    Now I'll tell ya my experience with JBM
    was working with my 6.5 gibbs, had all my load testing done but my PDA hit the skids, so the night before deer season I went to the sight and printed me out a range card, using my tested data. the next morning I took a whitetail at 787 yards using that range card. its a mathematical formula, feed it the right info and it'll be right.
    RR