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

 
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Old 06-13-2001, 01:05 AM
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my drop chart doesn`t match at 500yds and beyond?

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?
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Old 06-13-2001, 03:53 AM
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Re: my drop chart doesn`t match at 500yds and beyond?

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 ]
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Old 06-13-2001, 06:17 AM
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Location: Arco, ID 83213
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Re: my drop chart doesn`t match at 500yds and beyond?

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
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Old 06-13-2001, 06:34 AM
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Re: my drop chart doesn`t match at 500yds and beyond?

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.
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Old 06-13-2001, 06:48 AM
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Re: my drop chart doesn`t match at 500yds and beyond?

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
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Old 06-13-2001, 08:02 AM
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Re: my drop chart doesn`t match at 500yds and beyond?

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 ]
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Old 06-13-2001, 10:14 AM
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Re: my drop chart doesn`t match at 500yds and beyond?

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
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