BC help

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Tikkamike, Jan 16, 2011.

  1. Tikkamike

    Tikkamike Well-Known Member

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    I went out today to see if I could figure out my bc. First thing I did was verify my muzzle velocity. I came up an average of 2754. Next I shot at 100 yards and got an average velocity of 2640 I didn't go Any further shooting over the Chronograph because I wasn't sure him much I was going to trust these numbers to produce my final BC.
    next I shot a 3 shot group at 200 and another at 300. All I had was a short box so I didn't think I had enough room to shoot 400 so I shot a 3rd group at 350. Once I had all this done I came home and used the jbm calculator to find the BC using the velocity I got just for kicks. And it told me. 809! I obviously didn't think that was right but I ran that number through the shooter ballistics program on my android. It said that at 350 yards I should haves 12.7 inches of drop. My drop was 10.5 inches at 350 and 6 inches at 300. Next thing I tried was putting in a BC of 770 in my program and I came up with the same numerals as I did with 809. Whichever leads me to believe the parameters of my expirament were too small to give me the precise info I am looking for. Can someone with some experience give me their opinion? Thanks.
     
    Last edited: Jan 16, 2011
  2. Ridge Runner

    Ridge Runner Well-Known Member

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    Re: finding my bc

    not an expert but heres what works for me

    sight in at 300 yards
    very carefully shoot a group at 400, 500, 600 (use a big target)
    measure actual drops
    open up your program and tweak the BC until your chart matches the drops

    to tune it even closer the next session shoot at 500,600,700 yards
    this method has worked well out to 1300 yards for both rifles that I tested it with.
    When I started I was shooting a 160 accubond, at 3575 fps, I was always 1.25 moa high
    after moving the BC from .531 to .62, I'm right on out to beyond 1K
    RR
     
  3. Tikkamike

    Tikkamike Well-Known Member

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    Re: finding my bc

    How do you guys think it would work to shoot say, a 10 shot group on a piece of plywood. And measure my drop and adjust BC accordingly. Then tweak as I expirament at different ranges? It seems like it would work to me. Opinions please!
     
  4. Tikkamike

    Tikkamike Well-Known Member

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    Anyone else?
     
  5. Topshot

    Topshot Well-Known Member

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    Tikkamike,
    Hard to work out a B.C. from close range observations of bullet drop. Better to do it at the longest distance available to you.
    This is what I would do.

    First calibrate your scope elevation turret. This will confirm that what you dial is what you get. Or what error is inbuilt into your scope turret.

    Then shoot at least five shot groups at three distances.
    (1) 100 yards,
    (2) Your maximum distance and
    (3) mid range.

    Use your measured velocity and your calibrated amount of dial up at each distance to calculate a usable B.C. based on (real) MOA dialed.

    You need real MOA to calculate a good B.C. but as you may be dealing with a turret with error, you will need to calculate a correction factor. i.e. if you dial up 30 MOA on your scope and you actually get 32 MOA then your correction factor for "Shooter" is 0.9375

    Then if your turrets are not quite accurate, use the correction factor on the "shooter" program to give you your corrected turret adjustments needed for any distance out to your measured and tested maximum.

    This sounds complicated but is easy to do with the "shooter" app. I have been doing just this same thing over the last few weeks with Hornady 225 grain pills out of my .338RUM and it works very well with this app on my HTC Android phone.

    Let me know if you need any further help.
     
    Last edited: Jan 16, 2011
  6. bigngreen

    bigngreen Well-Known Member

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    I made an 8 ft target frame and I sight in at 300yrds, to get me farther out, and then I shoot 3 shot groups and walk it out till I run out of target, using the same aim point, then just to make sure I have it all locked down I shoot the same ranges but dial the scope according to the come ups I directly measured, If everything is right I end up with a string of groups down the target then a cluster of shots on the aim point. It's a little over kill maybe but my data is locked in!
    I then use that data to tweak my velocity more than my BC, I've been using Litz BC numbers and I think they are more accurate than a crony and that has worked very well for me. Using Exbal I shoot 300 and 600yrds and if I can 800ish then do a trajectory validation and I'm GTG inside 1000-1100yrds without messing with the BC,s. Using Loadbase I seem to have to shoot a little more if I don't hit right on the velocity but after I get it down I can usually shoot to transonic without any issues.