G1, G5, G7 curves. BC's, Drop, curves and putting it all together

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Brent, Feb 23, 2004.

  1. Brent

    Brent Well-Known Member

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    G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    Just reading a post of 308 WIN's on another thread here, I figured I'd do a little typing and "maybe" help fill in some blanks that some may have concerning BC's, drag models and the like. [​IMG]

    Anyone, feel free to add to, question, or downright correct me here. [​IMG]

    In a nutshell, the different drag curves will take a trajectory curve and either flatten the arc, or the opposite.

    The G1 (SAAMI Standard Model) drag curve produces the least flat arc of the three commonly used models; the G7 is the flattest, with the G5 in between.

    What to look for:
    If you have a very accurate average MV, and 100 yard zero, then you get your drops out to 1000 yards or so, each 100 yard drop should perfectly match with a ballistic program from 100-1000yds using the G1 model. You will need to adjust the BC to fit the 1000 yard drop to match yours first, using your KNOWN MV and atmospheric conditions (VERY IMPORTANT).

    If it does not match, look to see if the arc is actually flatter in the middle somewhere than the "predicted" drop indicates, then choose the G5 and modify the BC to fit at 1000 yards again. Some programs will convert the BC for you, it must still match at 1000 yards though. Adjust as necessary because you know the other variables. If this is still not flat enough in the middle somewhere, move to the G7 model and repeat. One should very closely model what you are seeing in the REAL world.

    Each of the G1, G5, and G7 curves model a specific bullet shape, and some programs actually modify the G1 curve and come close to mimicking the G5 to G7 bullet form, some are far worse than other and do NOT tho.

    One curve will never fit all bullet shapes and this is fact. For the utmost accuracy in a program modeling your "actual" trajectory curve, selecting the appropriate curve is recommended.

    Atmospheric conditions and MV changes due to temp or fouling changes -
    The drop data is only valid in the conditions you tested it in, but once you have determined the BC, measuring and accounting for atmospheric pressure, wind speed and MV changes that alter trajectory and drift significantly at LR is necessary for first round hit capability, with very high confidence.

    This can be easy duty when using a a Palm computer with ballistic program in the field, along with a method to monitor wind and atmospheric pressure, such as a Kestrel 4000 or Sherpa.

    You must already understand where your MV will be at different temps and states of fouling, and if not, testing loads at different cartridge temps using a cooler, ice and thermometer to stabilize ammo temp for testing throughout a range session can provide the info very quickly.

    Actual drop determination -
    You must record MV, altitude and station pressure then correct it to sea level for the BP during this testing procedure in order to create a baseline condition from which to work from in the future, without this, results are virtually meaningless at a later date.

    Use the BC you have determined to be accurate, along with a drop chart that will keep you on paper out to 1000 yards.

    Aiming at the top edge of a 8' piece of plywood beginning at 300 yards on out to 600, with a very perfect 100 yd zero to start with will allow you to measure "actual" bullet drop with a tape measure. Make no turret adjustments; just let the bullets drop at each 100 yard increment on to 600 with 3-5 shot groups. I use freezer paper to cover the plywood with, and keep the horizontal crosshair on the top edge of the plywood.

    Record these drops, mark each bullet hole with a marker so not to be confused on subsequent shots, then go back and dial your come up for 600 yards and reshoot at bulls eye or target or some sort to verify.

    Now move on out to 700, 800, 900, and 1000 yards dialing and recording your MOA dialed in, and then the groups point of impact (POI) difference from point of aim (POA) that you held.

    You'll take these notes and record them as your baseline sight in conditions for "this" load.

    You can get the atmospheric pressure at sea level (BP) from here http://www.srh.noaa.gov/data/forecasts/AKZ111.php?warncounty=AKC170&city=Palmer
    Or http://www.w3.weather.com/outlook/recreation/local/99645 or somewhere else, but this is not pressure at altitude (station pressure), it's barometric pressure, which is pressure corrected to sea level, which your BC is based on also. Therefore, you must also know the altitude where the testing is happening if using BP to determine station pressure during testing.

    If you have a means to measure station pressure at your location, you must know the altitude you are at to convert this to calculate the BP.
    If you are at sea level shooting and there are very overcast conditions, you could be shooting in conditions with a density altitude of well over +5000 ft ASL, thus POI hitting MUCH higher than you would the very next day on a clear sky with a very possible density altitude of -5000 ft ASL, and hitting WAY low now.

    Trajectories at standard conditions work at standard conditions, and density altitude can swing wildly so, account for it. The Kestrel 4000 works wonderfully for this, and I am sure a few others do as well.

    You should also shoot at varying temps and pressures to verify how well your ballistic program is modifying your firing solutions at LR, verses what you are actually determining is needed. Once you get the BC and MV's all nailed down, most programs can do a great job of correcting for air density.
     

  2. Mikecr

    Mikecr Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    Sure wish I could just sit on the back porch & do all this shooting, thinking, calculating, and shooting!
     

  3. Brent

    Brent Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    Ditto! Pain in the A$$ packin up and unpackin to do it. My new place will have the room for the back porch and range!
     
  4. Mikecr

    Mikecr Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    I'd be happy if i had a program that would simply tell me what my new zero would be under any of these condition changes. Or one that would allow me to calibrate the curve.
    I zero here in NC. But hunt in PA. Wheres my zero when I get there?
     
  5. Mikecr

    Mikecr Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    [ 02-24-2004: Message edited by: Mikecr ]
     
  6. Quarterbore

    Quarterbore Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    Please explain how you got your corrected BC from G1 to G5 (EX..495bc @ G1 and .320bc @ G5)
    Is there a formula?

    Aslo MV? >meaning Mach Velocity is that velocity divided by 1126? (Vel./1126)
    Thanks for the help. [​IMG]
     
  7. JBM

    JBM Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I'd be happy if i had a program that would simply tell me what my new zero would be under any of these condition changes. Or one that would allow me to calibrate the curve.
    I zero here in NC. But hunt in PA. Wheres my zero when I get there?<HR></BLOCKQUOTE>

    Go to my trajectory calculation page (not the "basic" trajectory, the other one). You can calculate a trajectory in NC, then enter the calculated azimuth and elevation for this zero in the data page (to the left), adjust atmospheric conditions and uncheck the correct elevation. This will cause the trajectory calculator to use the entered elevation and azimuth.
     
  8. JBM

    JBM Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Please explain how you got your corrected BC from G1 to G5 (EX..495bc @ G1 and .320bc @ G5)
    Is there a formula?
    <HR></BLOCKQUOTE>

    Yes there is. I'm going from memory here, so take it with a grain of salt. You have to G5/BC5 = G1/BC1 so that BC5 = BC1*G5/G1 where G5 and G1 are the drag models calculated at the same mach number. This gives you a G5 BC at the mach number.

    <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
    Aslo MV? &gt;meaning Mach Velocity is that velocity divided by 1126? (Vel./1126)
    Thanks for the help.
    <HR></BLOCKQUOTE>

    I thought that MV was muzzle velocity. Brent?
     
  9. Brent

    Brent Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    JBM,
    Could you run an example with that formula, I can't make heads or tails of it for some reason.

    Convert a .680 G1 BC to a G7 BC if you will.

    308win,
    I've got two programs that will convert the BC's, RSI Shooting Lab is the better one http://www.shootingsoftware.com/ Oehler Ballistic Explorer is another.

    MV is muzzle velocity the way I've know it. Max, where the hell are you? Come answer this one! [​IMG] Mach number abrev... is ???

    Mike,
    Blaine Fields has a program called "Precision Shooter's Workbench" that uses a "deceleration constant", or DK. It allows you to use the published BC and your MV, you just change the DK number to modify the trajectory curve independantly until it matches your drops. It works very well. http://www.precisionworkbench.com/2.htm

    The Exbal Palm program has a sight in condition page, and a field condition page to deal with altitude, density altitude, temp or MV changes. There's another way to skin this cat though. Compare the bore line drop at your conditions before you leave (on a ballistic program), verses the altitude at your hunting location to see how far your 100 yard zero will shift.
     
  10. Mikecr

    Mikecr Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    Yeah I think the constant they are talking about equates to Pejsa's retard coefficient rate. Will mess with it.
     
  11. JBM

    JBM Well-Known Member

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    Re: G1, G5, G7 curves. BC\'s, Drop, curves and putting it all together

    To convert a G1 BC of .680 to a G7 BC, find the values of the G functions. G1 is 0.359 and G7 is 0.169. You didn't specify a mach number, so I used 3.0. Then BC7 = 0.680*0.169/0.359 = 0.320 (at mach 3.0).

    Note that I'm using G function values calculated from CD vs. Mach number tables provided by Bob McCoy at ARL. They don't agree completely with the values published in Hatcher's notebook, etc. I'm also using an ICAO atmospheric density instead of the older Army Standard Metro value.

    The idea here is that the CD is the same no matter what drag function you use (it's the same bullet). That's all the equation G1/BC1 = G7/BC7 is -- a statement that the CDs are the same (for the same mach number and density). Then you just solve for G7.

    [ 02-24-2004: Message edited by: JBM ]