my drop chart doesn`t match at 500yds and beyond?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Brent, Jun 13, 2001.

  1. Brent

    Brent Well-Known Member

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    I`m shooting a 416wby in a Ruger mag. Scope is a Leopold 3.5x10x40 I`m using 400gn Barnes XLC`s over 115gns of RL22 and GM215M primers w/ oal 3.755". Velocity is 2550 producing .300"-.700" groups verses 1.7" at 2650 fps.

    Advertised BC is .546 and at 2550 fps bullets hit as follows.

    2.25 moa at 200
    5.25 moa at 300
    9.00 moa at 400

    14.00 moa at 500 this is where things deviate
    19.25 moa at 600
    24.50 moa at 700
    31.75 moa at 800 was 15" low still ( 3 shots )

    The chart goes like this (JBM and LOAD FROM A DISK programs )

    2.31 moa at 200 yds
    5.39 moa at 300
    8.94 moa at 400
    12.89 moa at 500
    17.30 moa at 600
    22.20 moa at 700
    27.65 moa at 800
    33.69 moa at 900
    40.44 moa at 1000

    If I change the velocity to 2450 or something lower like that to coinside with the 500 to 1000yd moa end that puts my 100 to 500yd shots high on the target.

    So after 150 dollers on two boxes of bullets and 1.75lbs of powder I have an accurate chart for this bullet, etc. but am left wondering what variable would cause this.

    Is the BC changing around 400yds because of velocity dropping, and if so do the programs account for this? If I change the BC to .350 after 500yds the table falls real close to mine.

    Any insights on this before I go back to 1000yds?
     
  2. Dave King

    Dave King Well-Known Member

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    Brent

    I don't know the answer but I suspect the BC is lower than advertised and the velocity is pretty low at the further distances. Warren is a whiz at these things and Darryl probably has a good deal of experience with this sort of thing. I believe they'll need more info to help out.

    What was the temperature during these shots? What is the altitude where you shoot? Are your shots on the horizontal or are you shooting at a bit of an angle (up or down) and lastly, what's the prevailing wind speed and direction (clock method).

    [ 06-13-2001: Message edited by: Dave King ]
     

  3. Warren Jensen

    Warren Jensen Well-Known Member

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    Brent,

    As Dave said, we need some basic information. I need all the things Dave asked, plus what is the twist of your barrel? Also, how reliable are your chronograph readings. Have you calibrated it? Have you used other chronographs to check it? What shape do your bullets print on the target at the various ranges that you listed? Very few bullets will print perfectly round holes at all ranges.

    Thanks
     
  4. Warren Jensen

    Warren Jensen Well-Known Member

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    Brent,

    After rereading your post I have a couple more questions.

    When you list MOA are you converting for true MOA which is 1.047" @ 100 yds?

    You list a trajectory at 500 yds. and further that is printing lower than expected. ( Plus I don't understand the "31.75 moa at 800 was 15" low still") Is this what you mean? Because if this is true then you would have to increase the muzzle velocity to raise the long range impact points, not lower it.

    Help me out.
     
  5. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Brent

    We do need the temp.& altitude you are shooting at but, you chart is falling fairly close to my Oehler ballistics program.

    I too, think it is a lower BC then advertised, plus make sure the chronagragh is reporting the speed accuratly.

    Your sight height should probably be programed in at 1 1/2" above the bore on that rifle also.

    Rerun the program using a BC of .450 and see the results that are given.

    Just a thought as I have seen many bullet manufacturer's report higher BC for their bullets then what is a real figure.

    Darryl Cassel
     
  6. Warren Jensen

    Warren Jensen Well-Known Member

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    Lets talk theoretical here. I don't know if this is what is happening to Brent's bullets and trajectory, because I don't have enough information, yet. But, this is typical of what happens during long range bullet flight.

    A bullet that is not fin stabilized requires spin, or rotation, to attain gyroscopic stability. The amount of spin necessary to keep the bullet point nose forward can be very precisely calculated. When insufficient spin is imparted to the bullet, and it has any type of ogive and not a wadcutter, the oncoming air will cause the overturning moment to overcome the gyroscopic stability and the bullet will tumble nose high over backwards. It will continue to tumble. When the spin is just sufficient to stabilize it, the bullet will fly with it's longitudinal axis directly into the oncoming air. When the spin is more than sufficient the bullet will start to precess and yaw so that it's longtudinal axis is high and to the right(for right hand twist) of the oncoming air, and the nose will be drawing little circles in the air. The rate of spin that the bullet has that is more than the minimum necessary to keep it stabile will cause this precession and yaw to increase.

    Precession and yaw decrease the projectile's ballistic efficiency, the drag goes up and the effective BC goes down.

    Rotational drag is always less than linear drag, except in the case of bullets designed for Balanced Flight. The bullets we are talking about here will always have less rotational drag than linear drag. That means that the spin rate will decrease less rapidly than the velocity. Consequently, as flight time increases the bullet will be continuing to spin faster and faster than is necessary for it to maintain gyroscopic stability. Hence, the precession and yaw will continue to increase. The drag will be increasing and the BC will be going down. This is called BC decay and is very common for bullets that are not either designed for long range flight or are overspun at the muzzle. Sometimes this is referred to as "overstabilized", which is a misnomer, because as this condition continues the yaw will become so exaggerated that the bullet will become unstabile and tumble. What it is doing is spinning too fast. It also is why only properly designed long range bullets will decelerate back through transonic in a stabile and predictable manner. The collapsing shock waves of transonic phenomena will destabilize any bullet with exaggerated yaw.

    Very, very few bullets exhibit constant drag coefficients throughtout their flight. The concept that these BC variances fall into velocity brackets is an oversimplification of the physics involved, and leads to common mistakes in trajectory calculations.

    Most manufacturers (myself included) publish BCs that accurately describe the bullet's flight at the muzzle. It is the most accurate and predictable method. To accurately predict your bullet's trajectory from your rifle I would need to know not only the true velocity and twist rate, to the tenth of an inch, but the engraving characteristics, # of lands, depth, etc., and the muzzle exit yaw numbers.

    Or you can shoot your rifle at the various ranges, using the published BC as a reference, and see where the bullet is actually flying. This is the best way to determine what your rifle, with your load, is doing.

    [ 06-13-2001: Message edited by: Warren Jensen ]
     
  7. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Hello Warren

    Very good ballistic theroy and I was certainly not refering to your bullets at all.

    I have seen other bullet manufacturer's rewrite and republish BC at a later date and they suddenly became more or less then the original publishing.

    True, if you use the wrong twist that should be matched to the bullet, you will have downrange flight problems. Have seen it happen many times.

    To help Brent out, I only suggested dropping the reported BC and then see how close to the program he could come for his drop chart and for THAT rifle. If his velocity is true as stated, this is one way to use his program and rifle for longrange shooting.

    Sometimes you have to match the program to the rifle productivity in some cases instead of the other way around. Too many variables enter the picture.

    Received your package and very impressive.
    We will talk more about it later.

    Darryl Cassel
     
  8. Brent

    Brent Well-Known Member

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    Thanks for the time and help everyone. I`ll try to fill in the blanks and clarify some choppy statements, tried to be brief, sorry for the confussion.

    Temp +69 deg F
    Alt. approx. 150' above sea
    horizontal
    wind 10-20 at 11 o`clock (12 being muzzle end)
    1 in 14 twist

    I`m useing two different Crony brand cronys at 15', and both read between 2525 and 2610 with variances as much as that between them on the same shot. And top of that one will be faster one time and the other the next.
    I think these are ball-park brand cronys, wish I had bought a good Oehler model.

    Bullets print perfect holes at all ranges in paper and plywood backer with no evidence of elongation and if I remember correctly the programs indicatd quite a slow twist rate of between 19 and 21 depending on muzzle vel.

    The first moa chart is exactly the mark set at on the leopold dial that my bullets hit dead center for each distance indicated. The second chart below that is the exact moa the two programs I use come up with, based on 2550 fps with a .546 BC.

    But the main reason I used this chart at this vel. and BC is not just that my crony said 2550 but I feel more importantly that the moa adjustments for the first 400yds matched exactly those of mine.

    I was unable to duplicate this 400yd path with any manipulation to MV or BC. It seems no mater what I do to MV or BC or combination of both on the program, if it matches perfectly to 400yds it is always flatter at 500 and beyond than my groups print.

    What I ment to say on the 800yd group was, when I shot my 800yd groups dialed in at the 31.75 I suspected I needed the group printed 15" low and 10" group. So on the program if I entered in 2400 MV thinking my crony is lying to me, I find that would be more like it.

    Only one problem, that chart indicates my bullet should not be flying as flat as it apparently is by far at up to 400yds if it were going that slow. It would be printing several inches lower at my setting of 2.25, 5.25 and 9.00.

    The main reason I try to find a drop chart on the program matched to my known drops to 400yds is just to establish a path my bullet is following for the further distances to keep them on the paper.

    One more observation I`ve made correctly I hope is that if my scope is not calibrated for true moa and just 1" at 100yds instead, I would indeed be needing more clicks at the greater distances to make up for this especially if it is slightly under an inch.

    What is the most exact way to measure this in your experiance?

    I checked out the chart with a BC .450 and it seemed to be real close at 700yds but seems 800yds is SEVERAL inches off still.
    I`ll go to the range again and check a few things out. I was going to go back to 900 and 1000yds but looks like my scope has about 34.75 moa maxed out with a .025" shim already under the rear base.

    I wonder if like you say Warren, the bullet might be "overspun" and BC is falling off at distance as a result. What you say Darryl about matching the program to the product hit the nail right on the head. Thanks again for the quick response with all the help and suggestions.
     
  9. Dave King

    Dave King Well-Known Member

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    I e-mailed Leupold tech service a while back about the internal adjustments on thier scopes. The reply was that ALL thier scopes are true MOA adjustment. (I post this not because I automatically believe them but because it's what they state.)
     
  10. Warren Jensen

    Warren Jensen Well-Known Member

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    Brent,

    Yes, based upon your observations you do have BC decay, but you now know where your bullets will print at the various ranges. You can work with that.
     
  11. Brent

    Brent Well-Known Member

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    My own charts will work just fine. Something Ken H. asked on another thread was how accurate are these Bushnell rangefinders? Well thats a possibility, my YP1000 may NOT be as accurate as as I thought.

    It sure is repeatable and my range is set up based on the rangefinder, what I use in the field, not a tape, etc. If calibration is off just 2,3 or 4 yds every 100yds, multiply that out at 500+ yds and it is a real problem with trying to match it to charts for true yardage. I`ll let you know what I find when I measure things out.
     
  12. Ken Howell

    Ken Howell Member

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    FWTW, a couple of echoes and cobwebs from the void in my gourd. If you were here, we could run some loads through the acoustic target and get a better grip on the de facto, field-derived BC. (I've heard industry people, asked how they get their BCs, say "We figure 'em as close as we can, then we look at what the competition claims." That has always filled me with about as much confidence in their figures as there's nourishment inside a ping-pong ball.)

    Also, the equations in your software may be the usual, traditional equations that have worked so well, for so long, because we small-arms riflemen never put 'em to the strain that your shooting puts on 'em. I asked the late Homer Powley what value there was in Art Pejsa's equations, and he said that the Pejsa equations are minutely more accurate than the traditional. I've heard users say that Pejsa's equations are noticeably more accurate out to 1,000 to 1,200 yards. I don't know anyone who's tested 'em farther out than that.

    I may have just wasted your time. I haven't checked any of this out, so I'm just throwing somebody else's cats into the burgoo. Strain the hair out for yourself. Hope I didn't ruin the pot.