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B. S. Ballistic coeffceints

 
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  #1  
Old 10-31-2010, 08:34 PM
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B. S. Ballistic coeffceints

Hi I am a newbie to this forum because the other guys could not help out. I shoot a 7 rem mag with a 160 gr nosler accubond with a bc of .531. What I found is this bc is not even close I got a bc of .39. Could this be true. are they just trying to sell bullets so they lie about it or is my figuring wrong. I used the seirra program and in order to be on target at 1000 yards in my environment this is where the bc has to be.
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  #2  
Old 10-31-2010, 08:41 PM
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Re: B. S. Ballistic coeffceints

I would say your figuring is wrong. I have only used the 160 accubond out to 300 yds, but it is spot on. I haven't heard of anyone else having to use a BC that low. To check go to the JBM website and see if they have the 160 accubond listed if there is (Litz) after it this is one that Bryan Litz has accurately measured.
You might want to check your scope click values if you haven't already.
By that I mean clamp the gun/scope in a vice and see how much the reticle actually moves when you dial in 10 -20 MOA.
Can you post the data you used?

Stu.
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Old 10-31-2010, 08:46 PM
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Re: B. S. Ballistic coeffceints

mfrs lie to sell products, oh pshaw...



47.2% of statistics are made up on the spot.
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  #4  
Old 10-31-2010, 10:27 PM
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Re: B. S. Ballistic coeffceints

I have shot them in a 7RUM and a 7 rem mag and the .531 is pretty close. I would check on your velocities first and them double check click value.

How did you check your velocity?
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  #5  
Old 10-31-2010, 10:55 PM
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Re: B. S. Ballistic coeffceints

either your;

velocity
zero
scope tracking
distances
atmosphere

or part, (or all) of the above has errors.

velocity - try another chronograph alongside yours on the same day next time, compare the 2, if they are both the same, then its most likely ok.

zero - obvious, but the distance your scope is zeroed at must be accurate, ie exactly 100yds not 107yds for example... AND your impacts @ 1000yds is much more important, the center of your group @ 1000yds must be in the center of your aiming mark before you really know how much you needed to dial. if your groups are large, then the center of this group can be difficult to determine accurately at long range. Once you think you have it centered, use a fresh target and verify it again @ 1000yds with AT LEAST 5 shots assuming a dead calm day.

scope tracking - most scopes have a significant amount of error in their "click value", even the expensive ones, but some are alot worse than others. To check how much error your scope has, shoot a 3 shot group at a dot low on a large target at your zero distance, 100yds is easiest for MOA scopes, 100m for mil scopes. Dial a healthy amount (say 25MOA or 8mils, more the better) of UP elevation on your scope, then shoot another group at the same aiming dot. This group will impact about *about* 25inches higher now assuming 100yds. Take a tape measure and MEASURE ON THE TARGET center of first group, to center of second group and then compare this measurement to what it *should* have been in terms of MOA or mils by converting the inches or cms to MOA or mils. Calculate your scope error from the difference. Again, make sure the distance your shooting at is exact, 100yds = 100yds true. Only 5% tracking error, = more than 1 MOA @ 1000yds.

distances- again, you need accurate distances, use a proper competition rifle range where the distances are measured out correctly, or a quality rangefinder if you have you own private setup (unless you want to physically measure out 1000yds the old fashioned way) The bigger the distance, the MORE CRITICAL this becomes. just 25 yds error at 1000yds from a 7mm = roughly 3 clicks. When you reverse calculate the BC, your calculating a trajectory thats 3/4 MOA flatter or rounder than it really is and you get a big difference in calculated BC.

Atmosphere - you need proper tools here; the humidity can largely be ignored, the temp is a little more important, but the pressure and altitude is very important. Make sure you got correct values here.

The BC will end up much closer to .531 if you check everything above and find your errors.

Last edited by groper; 10-31-2010 at 11:39 PM.
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  #6  
Old 10-31-2010, 11:34 PM
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Re: B. S. Ballistic coeffceints

Quote:
Originally Posted by Willys46 View Post
I have shot them in a 7RUM and a 7 rem mag and the .531 is pretty close. I would check on your velocities first and them double check click value.

How did you check your velocity?

I shot through a shooting croney cronograph about ten feet from the muzzle. I found velocity to be the same on different days. My velocity is 2952 fps.
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  #7  
Old 10-31-2010, 11:38 PM
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Location: Drumheller
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Re: B. S. Ballistic coeffceints

Thanks for your help guys. I guess it is back to the drawing board. Why can't things just be easy. Anything else you can think of would be appreciated.
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