Bullet BC

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by cabelas90, Apr 1, 2009.

  1. cabelas90

    cabelas90 Well-Known Member

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    Is there any way to figure true bullet bc other than with a croney? If so how would I do it and whats the math that goes with?
     
  2. Michael Eichele

    Michael Eichele Well-Known Member

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    No. You have to know your EXACT velocity to find it.
     

  3. cabelas90

    cabelas90 Well-Known Member

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    Thanks I guess i will just have to get a croney what model do you recomed? As of right now im shooting a 30-06 and a 204 ruger.
    Also in one of the emails you sent me you mentioned G1,G5,G7 drag models is there somewere i could find more info on this subject a book or something?
    ps i now i can be really hard headed but i appreciate you patence. I just want to learn as much as a can and have a better understanding!
     
  4. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Hey Zach,

    Easiest thing I can recommend for you here is to pick up a copy of "Hatcher's Notebook" from Stackpole press. There are a complete set of drag tables, along with several examples of how they're used. You still need to know velocity at two different points, and that's absolutely required to calculate a BC. The tables in the book are Ingalls', which can be used almost interchangeably with the G1 currently in use for US sporting bullets and ammunition. There's also an abbreviated copy of the Coxe-Bugless charts for estimating ogive values and ballistic coefficients by this shape. Not very precise, but an interesting history lesson today. It's worth picking up a copy of Hatcher's Notebook anyway, and every serious shooter should have a copy on his bench.

    If you really want to get serious here, there's "Modern Exterior Ballistics" by the late Bob McCoy. It's available from Schiffer publishing, but it's a bit steep at close to $100 a copy. It will still require the same basic info here; known velocities from two different points downrange. Anything beyond this, such as methods based on shape, basically comes down to an "educated guesstimate."

    Hope this helps,

    Kevin Thomas
    Berer Bullets
     
  5. cabelas90

    cabelas90 Well-Known Member

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    Wow that is steep! is there a certian croney or set of croneys that would be good. Also could i use one croney set it up at the muzzle for say 20 shots then move the croney to the target and take 20 shots then take averege's for each to get a true bullet bc?
     
  6. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Hi Zach,

    Well, you could do that, but frankly, the results are going to be a highly suspect educated guess, nothing more. There's a lot of factors that come into play here, and chrono errors are always a part of that. This is mostly due to their set-up, screen separation distances, distance to muzzle, etc.. Add to this the errors induced by trying to reset the same screens at a different distance, firing another set of the "same" ammunition, and basically you've got a recipe for a real goat rope here. Not worth the time or effort to wind up with what would be very suspect (at best) data. The easiest way here would be to use something like the Oehler 43, which is capable of generating BCs and is a more or less self contained unit. Rather than do all this, I'd really suggest you pick up that copy of Hatcher's Notebook. It will explain a lot of this in pretty readable terms, and there's a wealth of other information in there as well. I think I've gone through about four copies over the years and read the covers off them, quite literally.

    Kevin
     
  7. cabelas90

    cabelas90 Well-Known Member

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    Ok thanks ill look that up!
     
  8. 436

    436 Well-Known Member

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    This may help you.
    Go out with your load {bullet} you’ve picked.
    Chronograph your loads; and at the same time zero your rifle at 100 yards, minimum of five shot average for velocity verification.
    Then fire that same load at 1000 yards, acquiring 1000 yard zero from your 100 yard zero, counting the elevation adjustments to 1000 yards in clicks or minute, {Note; this is for LR rifles you may need to change your scope base up to 15 or 20 MOA and of course it will depend on the caliber and bullet your using as to whether your rifle can reach 1000 yards supersonic, because it “must stay” supersonic}

    Next go to your computer, bring up your ballistic program, plug in all the baseline information the on the 1000 yard zero, which would be an over lay on the program you originally ran with the programs B.C’s
    This “shouldn’t” match up with the factory B.C…. showing more elevation needed in most case’s; so increase or decrease the B.C on the program until you click’s/minute adjustments program match up with want you obtained in the field.
    Once they match’.., recorded the B.C used on the program you’re working with
    One of the other benefits is that your elevation scale in 100 yard increments should be very close to right on. Going back to 100 yards from your 1000 zero.

    I’ve never tried this at 500 yards, so I couldn’t say whether it would or wouldn’t work… correctly, it’s just math so guess it should’…, but being as I’m a long range shooter, I know it works at 1k. By the way, I use both; Load from Desk and Sierra Infinity to work the number.
    Good luck
    436
    Ps; I don’t think I left any steps out.
     
    Last edited: Apr 8, 2009
  9. Buffalobob

    Buffalobob Writers Guild

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    Life Without a Chronograph

    I can still remember the first time I saw a transistor radio. The first computer I ever ran a program on was a Univac with real vacuum tubes. I was in graduate school when I saw the first programmable calculator and it cost over $300 back then which would be about $1000.00 at the current dollar value. Of course I can also remember when we first got indoor plumbing.

    I have never yet found a good reason to spend $300 on buying a chronograph. It seems that the very few times I want one that I manage to get over that want in a few days. So how is it that a person can make it through life without a chronograph. Everyone will tell you that you must know your muzzle velocity in order to use a ballistic calculator to run off a drop chart. So how can you determine your muzzle velocity without a chronograph. Well, you have three choices in how to determine your muzzle velocity. First one is to just give a good guess. Second one is to send $40 and a chicken leg bone to the gypsy lady down in N’awlins and she will tell you what your muzzle velocity is. Finally, there is the old stand by of determining it experimentally just like we used to do before you could buy a chronograph.

    To determine the muzzle velocity without a chronograph it is necessary to not be afraid of working with numbers. You do not need to have a lot of formal education with math but you do need to be willing to spend about two hours of time with a ballistic calculator each time you develop a new load. If you hate numbers and were simply born with no gift for numbers then by all means go and get yourself a chronograph and go and find yourself something interesting to read because the rest of this is really going to be boring for you.

    To set the stage for this, first you should realize that a person seldom knows the exact BC of a bullet and that it is normal to have to experimentally determine the exact BC from a given barrel. Thus we are faced with two unknowns- muzzle velocity and BC. Fortunately we were not asleep during Algebra I and remember that two solve for two unknowns you need two equations. To develop two equations we need three data points. If we have access to a good 1,000 yard rifle range we can develop eleven data points if we wish so we are in really good shape.

    If you are working with a custom bullet which has no published BC then you can go to this website and develop a BC that is pretty close.

    http://www.uslink.com/~tom1/calcbc/calcbc.htm


    If you are using factory loaded bullet then you can visit their website and find their estimated muzzle velocity. If you hand load then your loading manual will provide you an estimated muzzle velocity which may have to be tweaked if you are using a longer or short barrel.

    So we now have a good estimate of the BC and of the muzzle velocity so without even getting out of our chair we can run a preliminary drop chart using a ballistic calculator such as JBM found at this address:

    http://www.eskimo.com/~jbm/cgi-bin/jbmtraj-5.0.cgi

    With our preliminary drop chart we are ready to go to the rifle range and dial our rifle in. Even if we have a chronograph we have to do this so this is not extra work. It is just part of the normal process.

    At the rifle range we will zero our rifle at 100 yards. Once we have done that we will adjust the turret to be “zero”. We will then dial up the clicks to be on target at 200 yards and we will fire a group. It does not matter if it is a little low or high. Simply record the amount high or low in inches on your preliminary drop chart or a separate piece of paper. Dial up and shoot a group at 300 yards and record the amount high or low. Continue to do this at every 100 yards out to 1000 yards. If your preliminary drop chart is too far off then you can start to dial up the next amount plus or minus your last difference. At the end of the day you will have recorded your rifles real drops for every 100 yards out to 1,000 yards. Think abut what kind of rifle shot you are and do not expect perfect groups unless you are actually able to do that. You should also record the weather conditions for temperature, humidity, etc. you can look up the weather conditions on the internet if you wish. All of your drop data and weather should be carefully recorded in your loading manual or your computer or your rifle log because you will be needing them as time goes by.

    So now you have a whole series of real data points starting at your rifle barrel at “0” yards to 1000 yards. Unless you are very expert with math, I would recommend you get a clean sheet of paper and go down the page with 11 rows of yardages starting with zero and going to 1000 yards in a second column write the real world drops for each yardage. The real drop is the preliminary drops sheet number adjusted plus or minus the amount your group was off at that yardage. For example if your drop sheet said 8 MOA at 600 yards and your groups was 2 inches low (2” divided by 6 = 0.33) then your adjusted drop would be 8.3 MOA at 600 yards.

    Now comes the math work. We will be working with our ballistic calculator “try” different BCs and different muzzle velocities until we get it to duplicate our real world drop data.

    Enter the weather data that was observed the day you shot your drops. Look at your real drop data and see if your groups were always low. If they were low then it means your estimated velocity and or your estimated BC is too high and you will need to lower the number you enter into the ballistic calculator. If you were always high then it means your estimated MV or BC were low and you will need to increase them. Most often the situation will be that the your groups were high (or low) up to about 700 yards and then were low (or high). In other words, the groups were not consistently high nor were they consistently low and this means that your estimated MV is too high and the estimated BC is too low or vice versa.

    So being by trying slightly different muzzle velocities of maybe 50 fps and see if the calculated values begin to come closer to your recorded real values. Copy each set onto your sheet of paper with the value of the MV. Once you have come as close to the real data as you can with MV start changing the BC to try to adjust it closer. Once you have it closer with a new BC go back and begin again adjusting MV with the new BC until things are closer and you will now have an even newer MV and be even closer to your real world data. Go to BC and try one more time to adjust it even still closer. When you get that done you now have a fairly accurate MV and a fairly accurate BC. When trying to decide how close is close remember that the critical issue is accuracy at the longer ranges. Make sure that your data is matching well from 500 yards on out to 1000 yards because shooting at a deer under 500 yards allows you to have a lot or room for error but you do not have so much room for error at the longer ranges. Also remember what I said about you being a sloppy shot and not an Olympic gold medalist. Your longer range groups that you shot might not be totally exactly centered because you just aren’t that good of a shot. So it is possible for there to be some wiggling around of the real data. My general level of accuracy for a three shot group is about 0.5 with an accurate rifle. So I would expect my group centers to move around about 0.25 MOA plus or minus from where I wanted them. If there is a wind then they will move around even more. You will not get a perfect fit.

    If you had a chronograph you would still have had to use the ballistic calculator and compare its output against your real drop data. You would have had to adjust the BC until you got a good fit. Without a chronograph it is some extra playing around with the ballistic calculator but it is only an hour or so. After you have been through the routine with a few rifles or a few loads you will begin to realize that you can get by with fewer data points. I like to have 100 yards, 300 yards, 500 yards 800 yards and 1000 yards. That will give me sufficient data points to get a good fit. You cannot get by with less than three data point and one of them should be beyond any distance you intend to shoot at an animal.
     
  10. Savageman69

    Savageman69 Well-Known Member

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    a tip all published ballistic coefficient figures are published by the manufacturer using the g1 model for drag so the best way to get an accurate estimate is to measure, a sample 15 or so of the ammo you usually use then find a ballistic coefficient calculator,input the measurements and there you go.

    i am currently putting together a chart for .22 wmr ammo i have measured about 5 different brands of ammo but i currently cant afford to buy every brand so i dont know when it will be done.

    if i could enlist some of your guys help.
    i have measured CCI 30gr vmax poly tip
    CCI 40gr Hollowpoint
    and fiocchi 40gr jacketed soft hollowpoint
    but i hav'nt really bought other brands
    if anyone has a micrometer and .22 mag ammo of a different brand or grain
    i could use the measurements all i need is the

    1 the leangth of the curved section of the bullet basically where the cartridge meets the brass to the tip

    2 the diameter of the tip measure the flat surface of the tip. and measure more than one preferably 15- or 20 because they all will probably deviate about .010 on all of the measurements just because thats mass production

    please only send me measurements from a digital caliper because the dial ones usually dont have an increment smaller than .050-.1"

    if you dont have a caliper and have some ammo and are feeling nice and would like to send me a few i would really appreciate it. PM me with anything you have for me