I am SOOOOOO confused. HEEEEEEEELP!!!!!!!!!!!!!!!!!!!!!!

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Michael Eichele, Aug 18, 2005.

  1. Michael Eichele

    Michael Eichele Well-Known Member

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    I was doing some testing resently. Here is the scenario:

    Custom 300 RUM (.333 ND) 27.25" Lilja #7 straight taper, 12 twist with a muzzle brake. Night Force NXS NP-R2 3.5-15x50mm. 95.0 H50 BMG 178 AMAX at 3100 on the nose. 300 yard zero. Dead nut zero I might add. A reliable chrony was used to determine 3100 FPS. This load was also chronographed during multiple sessions on different days. When I went to test this load at 600 yards something interesting happened. The bullet drop at 600 yards was only 37.7" The conditions at the time were 29.61" hg. 26 degrees F. 88' 77% humidity.

    Now the published BC for the 178 AMAX is .495 or .496. Most shooters here will agree that on the G1 curve out to 600 yards the BC is more like .515 to .525. Using .525 I have to enter 3440 FPS into the calculator to see 37.7" at 600 yards. If I use 3100 FPS the BC has to be 1.050 to make 37.7"

    The method for determining drop was this:

    NP-R2 = 2 MOA per hold over. 3 hold overs were used to hit dead nuts at 600 yards. 2 MOA = 2.094" * 3 bars = 6.282 * (600/100) = 37.692"

    Real world bullet drop should be closer to 49"

    I can see being a few inches or .05 in the BC, but these numbers are outragous.

    Any ideas???

    I forgot to mention that the hold overs in the reticle have been proven to be an actual 2 MOA
     
  2. Buffalobob

    Buffalobob Writers Guild

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    Are the stadia a constant 2 mils at ALL magnifications? Or is it only 2 mils at a specified setting?


    Subtension on many scopes changes with the power setting.
     

  3. Mysticplayer

    Mysticplayer Writers Guild

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    I have no explaination for you accept that I share the same amazing performance. I shoot a host of Hornady SST and Amax bullets in various cals. Velocities verified with chronies, scope adjustments verified at 100yds on graph paper.

    My 6.5 140gr SST flies stupidly flat and have had some superb results with the 165gr 30cal SST. The 30cal 155gr Amax fly way flatter then any other HP match bullet of equivalent of heavier weight from my 308.

    Just started playing with the new gen 162gr Amax and 150gr SST in 270. The 270 shows more normal come ups but the 162gr 7mm Amax is looking pretty flat. got some old gen 162gr Amax and these are even longer and look much more aero.

    Historically, the 6.5 140gr Amax had a very high printed BC number and probably flew better then that. Then they redesigned it to make it way less aero. Now the SST has that old shape and again flies really well. I wonder if the 6.5 Amax will be changed to its former glory?

    I just go with the flow and figure that something in my gear or measuring is off. But then again, could just be magic...these poly tipped bullets fly well.

    Now the Interbonds are even longer for their bullet weight.

    Jerry
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    That is a very good point BB. Yes, my hold over values will change with power. The tests were done on 15x, which is where my reticle = 2 MOA per hold over.

    Jerry, I am glad someone else shares this phenomenon. I have run the numbers every which way and just cant figure out why it shoots soooooo flat. 10" at 1000 isnt much, but 10-12" at 600 yards....Thats a HUGE differance. Oh well, I will just take my rediculously flat shooting load and be happy with that. (Its my sheep load)

    The funny thing is that when I shoot 155 AMAX's out of the same rifle at 3600 FPS, they fall at 600 yards VERY close to what the calculator sais they should based on a BC of .474, which is what it was in the real world in my 308. The 178's however WAY out perform the 155 AMAX.

    Thanks guys
     
  5. daveosok

    daveosok Guest

    Try using the G7 from JBM's web site .
    I entered all your data except Altitude which you did not state and my drop came up with 37.7 @ 600 yards dead on the nose.
    This was using your 300 yard zero and the BC of .495. The G1 function is a broad general usage function. The Amax is completely different and requires a different G function.
    <font class="small">Code:</font><hr /><pre>Range Velocity Energy Momentum Drop Windage Lead Time
    (yards) (ft/sec) (ft-lbs) (lbs-sec) (inches) (inches) (inches) (sec)
    0 3103.1 4703.7 3.03 -1.5 -0.0 0.0 0.000
    100 3010.4 4426.8 2.94 2.9 0.0 0.0 0.098
    200 2918.9 4161.9 2.85 3.5 0.0 0.0 0.199
    300 2828.8 3908.7 2.76 -0.0 0.0 0.0 0.304
    400 2739.9 3667.1 2.68 -7.8 0.0 0.0 0.412
    500 2652.6 3436.9 2.59 -20.3 0.0 0.0 0.523
    600 2566.6 3217.9 2.51 -37.7 0.0 0.0 0.638
    700 2482.2 3009.7 2.43 -60.4 0.0 0.0 0.757
    800 2399.4 2812.1 2.34 -88.8 0.0 0.0 0.880
    900 2318.1 2624.9 2.26 -123.1 0.0 0.0 1.007
    1000 2238.5 2447.7 2.19 -164.0 0.0 0.0 1.139 </pre><hr />
    My MOA chart says
    <font class="small">Code:</font><hr /><pre>______________________Yards________________________
    _______100____200____300____400____500____600
    MOA __1 MOA 1.04 2.094 3.141 4.188 5.235 6.282
    ____2 MOA 2.094 4.188 6.282 8.376 10.47 12.564
    </pre><hr />
    You have three moa lines but multiplied it by 2.094 instead of the moa valua of 2 moa at 600 yards which would be 3*12.564 or 37.692 is what is actually being covered in distance at 600 yards. Which is dang close to the drop I got using the JBM calcualtor on a G7 function.
    I hope this helps you out some.
    You got it right but went about it a different way than I did [ QUOTE ]
    NP-R2 = 2 MOA per hold over. 3 hold overs were used to hit dead nuts at 600 yards. 2 MOA = 2.094" * 3 bars = 6.282 * (600/100) = 37.692"


    [/ QUOTE ]
    Dave
     
  6. craigp40

    craigp40 Well-Known Member

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    Dave, I'm assuming you know how to properly choose the G# for a given bullet. Can you explain what this is and how you determine which one to use? I'm not really familiar with it, but I'm guessing it has to do something with how the BC changes over the course of flight, similar to how Sierra publishes their data.
     
  7. Michael Eichele

    Michael Eichele Well-Known Member

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    I hadnt tried the G7 curve, but I can see that when the G7 curve is used, its MUCH closer.

    I didnt think that at 600 yards the differance between G1, G5, and G7 would be that differant. I even have 2 personal calculators that utilize the G7 curve. Thankx for pionting that out. I learn something every day!! /ubbthreads/images/graemlins/laugh.gif
     
  8. jb1000br

    jb1000br Well-Known Member

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    Michael -- last time this happened to me i was shooting over a drop in the terrain -- my explaination was LIFT caused by the wind direction -- wind was coming in quartering and following the terrain lifting the bullet on its way to the target.

    YMMV,
    JB
     
  9. Mikecr

    Mikecr Well-Known Member

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    Whats the G7 BC?
     
  10. daveosok

    daveosok Guest

    Well Craig here's a quote from a site that has what the G functions are for [ QUOTE ]
    G1.1 - Standard model, Flat Based with 2 caliber (blunt) nose ogive

    [/ QUOTE ].
    As you can see this simply will not work as it doesn’t describe the bullet in question.
    The G5 was [ QUOTE ]
    G5.1 - For Moderate (low base) Boat Tails - 7° 30' Tail Taper with 6.19 caliber tangent nose ogive

    [/ QUOTE ] [ QUOTE ]
    G6 is G6.1 - For flat based "Spire Point" type bullets - 6.09 caliber secant nose ogive

    [/ QUOTE ] and finally G7 [ QUOTE ]
    G7.1 - For "VLD" type Boat Tails - long 7° 30' Tail Taper with 10 caliber tangent nose ogive

    [/ QUOTE ].
    Now compared to a Berger 185 grain bullet and the A-MAX 178 grain bullet the differences are the boat tail is shorter, and not at the same angle, the shank length is very close to the same but the ogive is shorter but very close to the same radius.
    Since the G5 function didn’t match his drop data I switched to the G7, the G5 is more of what the A-Max looks like but the data didn’t correspond to the real world trajectory he stated.
    I placed a call to Hornady and they suggested using the G1 function, when I explained to the gentlemen that the G1 function had already been used and was not matching up with real world data and that JBM software matched very closely he said that’s interesting and I will pass that on to our ballisticians.
    For whatever that’s worth if the numbers match and match at more than one interval then obviously using the G7 function with his data is correct. When I say correct it’s not correct in the meaning of ruling out any other possibilities just that with his data, the JBM program and the G7 function used it was correct for his data. Keep in mind though that two points in the trjectory path are known, the 300 yard zero and the 37.7 inches of drop, the G7 function placed the bullet at exactly where he said his dropped to or very close so much so that it isnt worth aruging about.
    Mike the BC never changes between G functions what changes is the drag and how effective the bullet is at beating it.
    I used the same BC that Hornady claims from their web site.
    One key element here is we still do not know the altitude which may or may not provide an answer as to why the G7 function worked while none of the others didn’t.
    According to this site if you have a program that allows multiple inputs at different ranges it will calculate for you the BC at those ranges for the A-Max bullet the Bc2 through BC4 limits are all 1.00.
    Jb100br may be on to something also, however it is highly coincidental that 37.7 matched perfectly for the G7 function, the odds of the wind blowing the bullet upwards and matching exactly 37.7 is by chance I think but will not rule it out as a possible factor in the trajectory.
    JM2C
    Dave
     
  11. Mikecr

    Mikecr Well-Known Member

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    Hornady's BC claim is G1BC at standard metro or ICAO conditions. To use G7 drag curves, you have to enter known G7BC. For example if your Hornady's are really .525 G1BC then they are likely ~.255 G7BC. Using G1 BC with a G7 curve would give you low drop numbers -falsely.

    Altitude isn't needed because pressure was stated, but if you look closely you can see 88' was the altitude.
     
  12. daveosok

    daveosok Guest

    I used the G7 function on the JBM. Can you tell me if the JBM uses a G1 fucntion for all G functions listed and uses the G functions past G1 for curve like you said about the G1 with a G7 curve?
    I dont know if it does or not maybe you do thats why I am asking?
    /ubbthreads/images/graemlins/blush.gif I want to make a correction, I misread the data and entered 168 instead of 178 but re-entered the correct data and still came up with 38.6 inches of drop so I apologise for the .9 error in trajectory.
    My mistake once again but the data didnt change drastically. I would really like to know if the rest of the data from 100 to 300 and 400 to 500 match up with the trajectory table using the 178 /ubbthreads/images/graemlins/blush.gif bullet.
    Dave
     
  13. Mysticplayer

    Mysticplayer Writers Guild

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    dave, the BC of a bullet must change with G function or else the BC value is worthless. BC is a relative value of drag. The higher the number, the lower the drag given the same conditions.

    If the G7 function predicts the flight properly, then the G1 BC number must reflect this. The book BC is a G1 BC profile calculated number which doesn't agree with the real world. Do all listed BC numbers use the same G function? If not, the number is meaningless and misleading.

    As mentioned, I have had this very weird situation happen with select hornady bullets. In these cases, I must propose that the printed G1 BC is way too low. This is proven by the use of a G7 to predict flight.

    I have some old 162gr 7mm Amax that don't look anything like the new brand. The new brand has a listed BC of 625. The old stuff must be way higher. Yet, I bet the print value is the same.

    Jerry
     
  14. daveosok

    daveosok Guest

    Well then using the G1 on the JBM and entering a bc of 1.05 got me a drop of 38.3. Now you and I both know however slightly different the bc must be it aint 1.05.
    I understand what your saying but if the G7 function works then why not use it with the stated bc if the flight path matches his real world trajectory?
    Not everyone may be able to do this but if it matches his flight path then why go messin with it to find something else?
    That old addage "if it aint broke dont fix it" applies here.
    But like I said I would really be interested if the rest of the ranges matched the drop indication of the JBM program?
    Dave