changing ballistic computer input

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by arrow, Jan 30, 2012.

  1. arrow

    arrow Well-Known Member

    Dec 13, 2007
    I have a question about what to do when you put in all the atmospherics into exbal and you put your velocity reading from the chrono of your load and the bc of the bullet from the manufacturer and the computer is off by a little bit from acual data. Im aware about standard pressure and what to put in for the atmospherics, thats not what im asking about here. Im asking because i know there is more than likely going to be some error from the chrono and also the bc can be different for many reasons. So if the atmospherics are all plugged in right, than the velocity and the bc are the veriables. so how do you go about finding the right bc and velocity to plug in for that load?
  2. Broz

    Broz Well-Known Member

    Feb 3, 2007
    What I have found to be best after many years with many program is, At first enter your data and then go afield and shoot actual drops ever 100 or 200 yards out to the distances you intend to shoot. Then log all this data including the field conditions at the time of the shot. Then bring this data with confirmed drops home and start to fine tune yor curve with both BC and Vel until they match up everywhere with your program. Remember that since most of us shoot 1/2 moa rifles and your actual field groups will have this 1/2 moa included you may not get it perfect every where. But I have found that after shooting multiple groups at every test distance I can get most to agree with in .25 moa. Not a simple task, but you get back what you put in with first round accuracy at all distances. Usually the chono is the culprit, but I have had to fudge BC as well and when going beyond 1200 yards someties stepped BC's and low end velocity limits are needed.


  3. LouBoyd

    LouBoyd Well-Known Member

    Oct 15, 2007
    The basic process I'd use is:

    1. identify all of the parameters you have available in the ballistics program to adjust. . You didn't mention the scope height above the bore which is important. (that's the ceterline of the scope or sights vs the center of the bore) when zeroed. on the target. You also didn't mention if Exbal allows BCs to be adjusted over ranges of bullet velocity. I don't use Exbal.

    2. Make estimates of how accurately you know the correct value of each parameter. Leave the ones you're sure are correct alone. Single value BCs are just about always wrong (though not necessarily by an unacceptable amount) except at one velocity. You need to estimate how much the errors mght be based on bullet shape relative to the selected G model shape. That's not always obvious.

    3. Think about the effect each parameter has on the bullets apparent trajectory. For example, the scope height parameter will always appear as constant angular error away from the zero distance if it's not measured and entered in the computer correctly. Bullet drop is always proportional to the time of flight squared unless there is wind with a vertical component. Muzzle velocity will change the distance scale entire trajectory but it will not change the drop as a function of time. BC errors and air density errors have a similar effect on bullet drag and the shape of the trajectory but from the way the calculations are done only BC should be adjustable as a function of bullet velocity. Since air density is meaured frequently in the field it should not be tinkered with to make adjustments, but it's important that it be measured correctly. There are
    several ways to determine air density. With all of them you have to be aware of units, calibration, and zero offsets. Temperature, altitude, humidity, and storm conditions all affect air density.
    Make sure you have it right, but don't change it to correct to your observed shooting errors.

    4. Question your actual fired trajectories. Were the targets placed at the right distance? Were the atmospheric conditions the same as those you're trying to calculate? Did the terrain allow any up or down wind effects? If individual points in your observed trajectories seem out of place with the other points (at supersonic velocities) check them before blaming the computer calculations.

    5. Plot the calculated trajectory on the same graph with your fired trajectories. If they line up well with only a uniform increase with the square of the time of flight (use the computer results to get time of flight) then you should be able to just adjust the BC value to get a better fit. You could adjust the velocity, but If you've measured it accurately with a chronograph it's best not to. Muzzle velocity can also vary with temperature and so you want it to be correct for the conditions. It the error is linear with distance re-check the scope height but don't change it if it was measured correctly.

    If changing the BC value doesn't give a good fit you try a different G model if you think it might work. Some boattail bullets fit the G1 model better than the G7 though usually not. Its more about ogive shape than having a boattail. Even if you don't have a factory value use the G model try what has the most similar shape to your bullet and adjust the BC value. Use what gives the best fit. If your computer has an multi-segment BC model you can use them. Remember that the lower velocity entries have no effect until the distance where the bullet reaches the top velocity of that segment. Get the best match you can using a single BC over the first half (more or less) of the trajectory, then use the segments at lower velocities to adjust the calculate trajectory out to transonic range of your bullets. If it takes large changes in in the BC coefficient something else is wrong. Trying to match BC curves through the transonic range isn't worth the effort.

    If there are a couple of odd points in your shooting tests which just won't fit the calculations they're likely to from errors in the shooting tests, , not in the computer program.

    You don't want to adjust any parameter in the program which you may want to measure and correct in the field when shooting. That would make it too confusing to use. Also if the bullets don't seem stable don't group well, there's little point is bothering with this whole exercise.

    I'm interest to hear how others would do this. If the errors are small you can just "guess and tweak" the BC and likely get rid of most of the differences. If your velocity, scope height, or air density measurement are wrong then adjusting BCs will just screw things up worse.